This is a reworking of the "Add-on Vampire" Subrace made by HousestewsAndHomebrews. I made it a feat so that it can be used in conjunction with another subrace, as intended, and I also changed some of the abilities, effects, etc to be a little bit less/more punishing to fit what I wanted in my campaign. Feel free to do the same.

This is meant to be used as an add-on to an already selected race. Add this to a character you've already selected a race/subrace for should they become a vampire, or you wish to start as a vampire. 

You are afflicted with Vampirism.

Age

Your aging slows to a halt. You are unable to age any further than the exact age you were when you became a matured vampire.

Vampiric Disease

You gain the following effects/traits:

Vampiric Nature. You gain resistance to Necrotic damage. Your skin is deathly pale, the exact shade depends on your character's original ethnicity and skin color. You are now considered both an undead and a humanoid. You retain all of your normal traits of your normal race. You do not need to eat or drink regular food, and you only need to rest for 8 hours once a week to function normally. 

Vampiric Blood Lust. Your diet consists of humanoid blood. You need 3 rations of blood a week to survive; these rations can be taken all at once or spread out across multiple days or multiple humanoids. If you go more than 7 days without these rations you will begin to suffer one level of exhaustion for every day you haven't fed.

Superior Darkvision. You gain Darkvision to a distance of 120ft.

Unnatural Strength. You gain Expertise in the Athletics skill.

Harmed by Running Water. You take acid damage equal to twice your level if it ends its turn in running water. This acid damage ignores resistance and immunity to acid damage that may be granted by your base race.

Stake to the Heart. If a piercing weapon made of wood is driven into your heart while you are incapacitated in a place of rest, you become incapacitated until the stake is removed.

Sunlight Hypersensitivity. You take radiant damage equal to twice your level when you start your turn in sunlight and are not sufficiently covered. While in sunlight, whether covered or not you have disadvantage on saving throws. To be considered sufficiently covered you can be wearing any clothes that protect your skin from direct sunlight such as a large, hooded cloak.

Vampiric Strikes

Your teeth and nails have grown in length and sharpness. You may use your sharp teeth or claws as a natural weapon.

Vampiric Strike. (action) This is a finesse attack. Make a melee attack roll, and on a hit deal 1d8 + your strength/dex modifier Slashing damage, 

Bite. (bonus action) Make a melee attack roll, and on a hit deal 1d6 + your Strength modifier Piercing damage, and heal for 1d6 + your Constitution modifier or half the total of the damage dealt, whichever is greater (or if you're allowing your vampire pc's to be healed by healing spells such as Cure Wounds, just take half the total).

Kiss of Death

You sink your fangs into a creature, draining them of their lifeforce and fueling your own.

If the creature is unwilling once per short rest you can make an attack roll toward a creature in range. On a hit, the target must make a Constitution saving throw (Your DC for this is 8 + your Strength modifier + your proficiency bonus), and takes 2d6 + Your Strength modifier Necrotic damage, or half as much on a successful save. You heal for 1d6 + your Constitution modifier or half the amount of damage taken by the target, whichever is greater (or if you're allowing your vampire pc's to be healed by normal magic spells such as Cure Wounds, just take half the total). This bite counts as one ration for purposes of sating your Vampiric Blood Lust. 

The amount of damage and healing done by this attack increases by 1d6 at level 5, 2d6 at level 9, 4d6 at level 12, and at level 14 the die changes to a d8.

If the creature is willing then they automatically succeed the saving throw, and if you're not suffering from any levels of exhaustion then you can control yourself and stop feeding before any permanent harm comes to the creature. If you are suffering from exhaustion then you have no control over this, and your fate lies with your dm.

The Cure

While it is possible to cure Vampirism, it is extremely difficult. If you're choosing this feat because you want to play a Vampire PC, then chances are you don't want it to get cured by someone casting lesser or greater restoration, remove curse, or using their lay on hands. If curing your vampirism is a goal of your character, talk with your dm about how difficult you would like it to be, and what kinds of things you would want to include in that journey. Possible examples could be, hunting down the vampire that sired you, devoting yourself to a god in hopes of getting it's blessing to cure you, or gathering very rare magical components to create some kind of potion/spell.