Vampire Bite.

How come all Vampires in 5.5e use Bite as a Con save? I don't really see how your physical health determines how likely you are to get bite by a vampire. If it was a save for half damage, it might make more sense, but's no damage if you succeed. Confused on this, and I'm thinking of changing it to a Dex save.

Notice that the Bite action only works against a creature that is willing, Grappled, Incapacitated, or Restrained. The save isn't to avoid the Bite, you're already getting got so Dexterity wouldn't make sense. The save is to see how well your body can resist/endure having it's life essence (aka blood) drained. 

Good points, but here is my main gripe. With a Dex save, you could frame it as even though you're grabbed, you kind of still try and keep the fangs away from your neck. However, with Con, you take no damage on a success - not even piercing damage. Also, you have just the same chances of succeeding even if you are unconscious, which is another weird thing.

I think you’re still thinking of it as an attack to be avoided when as Davyd points out it only works on a creature that isn’t fighting back, they’re letting it happen so why would they make a DEX save to avoid something they want? The CON save to to see how badly you’re affected by losing a pint of blood, it’s not to stop it happening it’s to assess the result of something that has happened, and that wouldn’t change based on whether you were awake or not 

Quote from PPSS2022 >>

Good points, but here is my main gripe. With a Dex save, you could frame it as even though you're grabbed, you kind of still try and keep the fangs away from your neck. However, with Con, you take no damage on a success - not even piercing damage. Also, you have just the same chances of succeeding even if you are unconscious, which is another weird thing.

Let's address succeeding while unconscious—generally speaking Con saves represent your bodies resilience, which is not something you choose to employ. You can't choose to fight off a disease or succumb to it, poison will take you out just as effectively if you're awake or asleep. Con saves are, 99% of the time, your body doing the "work", not you choosing to like dodging an attack with Dex or resisting an assault on your sense of self with Cha.

For the first point (second), we can look at this in two different ways. First there's the game balance angle; the action is balanced around a full/none damage split rather than full/half, and how that maps onto the narrative be damned. However, if we want to map it onto the narrative, it's an abstraction of your bodily resilience. Remember hit points are not meat points and instead represent a narrative clock. Losing hit points represents how close you are to defeat, being taken out of the fight, not (just) how much damage your body has taken. The full/none Con save split represents "is your physical resilience enough that you can shrug this off without it increasing your chance of defeat". If you succeed, sure you've got two new holes and you look like you've had a bad shaving accident, but you're able to walk it off. A failure represents that draining bite dropping you below that threshold where it matters, at which point hit points start coming off (and you end up being permanently worse off thanks to the max HP reduction).

Quote from PPSS2022 >>

Good points, but here is my main gripe. With a Dex save, you could frame it as even though you're grabbed, you kind of still try and keep the fangs away from your neck. However, with Con, you take no damage on a success - not even piercing damage. Also, you have just the same chances of succeeding even if you are unconscious, which is another weird thing.

 For the piercing damage, how much are the fangs actually going to hurt? I think someone who's tough enough could just walk off those little stabs.