My group has decided that the next game that we play, they want to be vampires. I'm completely excited to do this, but I don't know how to work the mechanics to make them both their regular race and class, then still let them be vampires that are not under my control. I think that we decided that to start, they're only half way there, and they would still have to bite their first victim in order to become a full vampire. I've decided that there would still be a cure to it all, but if they don't want to actually turn, they still have to try not to succumb to the temptation to bite living creatures or something. Has anyone ever run a game like this, and does anyone have any clue how they did or might work the mechanics?

I'd refer you to the game Vampire the Masquarade for some game mechanics but you could do something like:

Blood Lust: Whenever they are around an open wound (within 5-10ft and at least 5hp of damage) they need ot make a Wisdom Saving throw or move towards the source of the blood, if they are within 5ft of the source they need to make a wisdom save at disadvantage or try to drink the blood (usually by grappling the target but they might just attack and drink formt eh corpse. Also need Wisdom Saves if you kill a creature to drink its blood or alignment shifts closer to evil.

Mortality: They start off with 10 mortality points, each time they drink froma creature they lose a mortality point, if they kill a creature to drink their blood they lose a mortality point, if they get to 0 mortality points they become a fully fledged vampire and it is not possible to be cured without a wish spell or divine intervention. If they get to 5 or fewer mortality points they make animal handling checks at disadvantage as animals shy away from them (certain animals asscoiated with vampires, rats, bats, wlves etc) may be exempt form this.

Back in 3.5e d&d there was a Vampire Spawn character option so you could also give bonuses such as:

Ability score increases: +2 Str, +2 Cha

Darkvision (60ft)

Skill Proficiceny in Deception, Investigation, Perception or Stealth

Vulnerabilities to silver and/or radiant damage

Blood Drain: If you grapple a target you can make a bite attack that deals 1d4 piercing damage and heals the vampire aliek amount of hp.

Thank you for your input! This was very helpful and insightful! There are so many things to check out to help me decide how to run it, and I definitely need to bring them down a couple notches from a full fledged vampire.

I made a sort of half-vampire race which you can take a look at here. People seem to like it. I see no way of making a full vampire playable race that won't be overpowered.

You can make a vampire class (or subclass) to solve the balance issues, though it will be a fairly high amount of work. The easiest thing as a subclass might be a Warlock patron (I mean, warlocks already have access to Charm Person, Enthrall, Hold Person, Suggestion, Gaseous Form, Vampiric Touch, the main things you still need for vampires are biting which can be a cantrip, shapeshifting to wolf or bat, and regeneration -- if you even give it to PC vampires).

I may poke at creating that subclass. It doesn't sound too hard.

I love the idea of the mortality point system. (Thanks, Rob76) You could make it a mechanic of the campaign that they are trying to harness vampirism without losing their soul. Each player can try to keep their Mortality where they like it, which will be interesting to see people embrace it to different degrees. Inching closer to full undeath comes with power, but drawbacks as well.  

Here’s what I came up with on my initial thoughts. Each time they drink blood, they instantly restore their full HP, but they lose a point of mortality. Spells like lesser restoration can give back a point of mortality (greater restoration, 2 pts). Drinking Blood requires 2 rounds of uninterrupted action dedicated to drinking the blood of a humanoid victim. Don’t want to make it too easy to “hit restart” in a fight that’s going badly. But still makes embracing a point of undeath an appealing strategy over cure wounds or healing potions.

Here’s the table I came up with using mostly traits on the Vampire Statblock, though with a trait or two that seemed to fit within the scheme of PCs as vampires trying to retain a shred of mortality.  Mortality Spectrum

10 MPs = Resistance to Necrotic Damage, Susceptibility to Holy Water
9 MPs = Superior Darkvision, Sunlight Sensitivity
8 MPs = Spider Climb, Forbiddance
7 MPs = Unarmed Strike, Harmed By Running Water
6 MPs= Nonmagical physical damage resistance, Sunlight Hypersensitivity
5 MPs = Shapechanger, Plants near you die within 1d4 hrs
4 MPs = Misty Escape rules take effect and drinking blood grants additional temp HP equal to their character level (these temp points CAN stack), vulnerability to radiant damage
3 MPs = Children of the Night, Insatiable Appetite (make WIS save when dropping to this point level. Restoration up to this level does not require this save.(=10+1/2 char level rounded down) or immediately seek more blood.)
2 MPs = Regeneration, make another Insatiable Appetite save at disadvantage (no save needed when restored up to this level)
1 MP = Legendary Resistance(excluding Insatiable Appetite), make another Insatiable Appetite save at disadvantage.
0 MP = Beyond Redemption. Lose control of your PC.

I actually like the class suggestion. If you make it a custom class, then you can have them "level up" in their Vampire class. You may have to impose certain requirements like, when you gain a character level, you can only level up in your Vampire class if you have done X, Y or Z (e.g., drink blood some number of times). As you get into higher Vamp levels, you could start adding those "less mortal" perks. So if you look at the 10 MP, 9 MP chart, you could flip it... Level 1 Vampire would be 10 MPs... Level 10 Vamp would be "Beyond redemption".

This would let any race do it, and even any "class," i.e. you could be a Fighter/Vampire multiclass, or a Rogue/Vampire, and so on.

I've got a level idea thingy going on another thread for hound archon, I'll see if I can dig out the 3.5e vampire spawn player race option and tweak it to 5e.

Quote from kcbcollier >>

I love the idea of the mortality point system. (Thanks, Rob76) You could make it a mechanic of the campaign that they are trying to harness vampirism without losing their soul. Each player can try to keep their Mortality where they like it, which will be interesting to see people embrace it to different degrees. Inching closer to full undeath comes with power, but drawbacks as well.  

Here’s what I came up with on my initial thoughts. Each time they drink blood, they instantly restore their full HP, but they lose a point of mortality. Spells like lesser restoration can give back a point of mortality (greater restoration, 2 pts). Drinking Blood requires 2 rounds of uninterrupted action dedicated to drinking the blood of a humanoid victim. Don’t want to make it too easy to “hit restart” in a fight that’s going badly. But still makes embracing a point of undeath an appealing strategy over cure wounds or healing potions.

Here’s the table I came up with using mostly traits on the Vampire Statblock, though with a trait or two that seemed to fit within the scheme of PCs as vampires trying to retain a shred of mortality.  Mortality Spectrum

10 MPs = Resistance to Necrotic Damage, Susceptibility to Holy Water
9 MPs = Superior Darkvision, Sunlight Sensitivity
8 MPs = Spider Climb, Forbiddance
7 MPs = Unarmed Strike, Harmed By Running Water
6 MPs= Nonmagical physical damage resistance, Sunlight Hypersensitivity
5 MPs = Shapechanger, Plants near you die within 1d4 hrs
4 MPs = Misty Escape rules take effect and drinking blood grants additional temp HP equal to their character level (these temp points CAN stack), vulnerability to radiant damage
3 MPs = Children of the Night, Insatiable Appetite (make WIS save when dropping to this point level. Restoration up to this level does not require this save.(=10+1/2 char level rounded down) or immediately seek more blood.)
2 MPs = Regeneration, make another Insatiable Appetite save at disadvantage (no save needed when restored up to this level)
1 MP = Legendary Resistance(excluding Insatiable Appetite), make another Insatiable Appetite save at disadvantage.
0 MP = Beyond Redemption. Lose control of your PC.

I like this idea but I think I might cap the feeding to something like drain blood and use that creatures Hit Dice to heal, so you use another creatures HD to heal instead of your own.

The below is taken from 3.5e Libris Mortis book for Vamapire Spawn player race (pg39):

Start by choosing a race/background/class  as normal, Vampire Spawn is an "aquired" template which means you could have it as your 1st class level or technically take it any time you leveled up (story and DM permission dependant of course)

lvl 1: Gain 1d12 Hit Dice, Gain +2 Str, +2 Cha, Cons score = 0 (in 3.5e undead did not have constitution scores), Darkvision 60ft, +2 bonus to Bluff, Hide, Move Silently, Listen, Search, Sense Motive and Spot checks, Vampire Weaknesses: Vampire Spawn are vulnerable to all attacks and effects that repel or slay Vampire, Gain Language Common if not already known, Blood Drain (grapple to deal 1d4 Constitution Dmaage and gain 5 temp HP that last for 1 hour), Natural Attack: Slam (1d4), Bonus Feat, +2 Turn Resitance

so converting that lot over would give you: +2 Str, +2 Cha, Darkvision 60ft, +2 Bonus to Decpetion, Insight, Investigation, Perception & Stealth skills (this could also be changed to proficiency or advantage on those rolls), Vampire Weakness, Gain Language Common.Blood Drain (grapple to deal 1d4 Constitution Dmaage and gain 5 temp HP that last for 1 hour), Natural Attack: Slam (1d4 Bludgeoning damage), Bonus Feat, Turn Resitance (gain advantage on any attempt to resist being turned/destroyed)

lvl 2: +2 Cha, +1 Natural Armour, Alertness

So that converts over to: +2 Cha, Base AC = 11 +Dex (this does not stack with armour but does with shield), Gain Alert as a bonus Feat.

lvl 3: Bonus Feat, Spider Climb, +2 Dex, gain +1d12 Hit Dice

So that converts over as: Always under the effect of the spider climb spell and gain +2 Dex, +1d12 Hit Dice

lvl 4: +2 Natural Armour, +4 Skill Bonus (replaces the +2 bonus noted at level 1), +2 Str

So that converts to: +2 Str, Base AC = 12 +Dex (non stacking with armour but does stack with shield), Skill bonus is a bit of a toughy, if you had the level 1 ability grant proficicency then you could up that to expertise or advantage, if you had it as the basic +2 onus then its now +4.

lvl 5: +1d12 Hit Dice, Fast Healing 1, +2 Wis, Lightning Reflexes

So that converts to: +1d12 Hit Dice, +2 Wisdom, LIghtning Reflexes: Gain Advantage on Dex based ability checks (including Initiative), Fast Healing: Heals 1hp at the beginning of each turn, if they take radiant damge or damage form holy water this ability does not function for 1 turn.

lvl 6: Bonus Feat, +2 Cha, Resistance to Cold and Electricity 10

so that converts to: Bonus Feat, +2 Cha, Gain resistance to COld and Lightning Damage,

lvl 7: +1d12 Hit Dice, +3 Natural Armour, +2 Int, Gaseous Form, Slam 1d6.

so that converts to: +1d12 Hit Dice, Base AC now = 13+ Dex (this does not stack with armour but does with a shield), +2 Int, Gaseous Form as per Vampire, Natural Attack: Slam attack now deals 1d6 Bludgeoning damage.

lvl 8: Domination, Energy Drain, Fast Healing 2, +2 Str, Improved Initiative, Damage Reduction 5/silver.

so that converts ot: Domination (use action to try to affect someone as per Dominate Person, DC to resist equals 8 +CHa + Proficiceny mod), Energy Drain: A creature hit by a vampire spawns slam attack must make a Cons Saving throw (DC to resist equals 8 +CHa + Proficiceny mod) or have their max HP total reduced by an amout equal to teh damage taken in addition to taking the damage, the vampire spawn gains 5 temp hp for each failed save,  Gain Dmage resistance to Bludgeoning, SLashing & Piercing Damage unless it is from silvered weapons, Improved initiative: You can add your choice of Int, Wis or Cha mod to your initiative rolls (stat to be chosen when level gained and once chosen cannot be changed). Fast Healing: now heal 2hp at the beginning of each turn unless damage caused by radiant or holy water etc...

***

All in all it would a tremndously over powered entity to play, on the up side of things you have to invest 8 levels to get all the abilities and only get 3d12 +cons mod HP, on the down side you get huge amounts of stat increases, a couple of extra feats of your choice, odds are you'll go first in any situation where initiative is rolled and ould likely go toe to toe with a lvl 8 barbarian in full rage.

That said, if the entire camapign is based around all players being vampires and struggling to keep their Humanity/Dwarvishness/Elfness etc then this won't be too bad as you'll end up trying to balanbce increasing your vampiric abilities at the expense of your class abilities.