like it says, i'll walk up to him in vampire form and he asks if i want the dark gift?

He wants to see if you remember that you're a vampire :cookie:

You have to understand that Vicente is old, even for a vampire. He forgets things. Or maybe he's not referring to vampirism.
If that's the case, you should :bolt:

I'm surprised to hear that (I've never played a vampire). I would have expected Vincente to recognize that and have some dialogue to support it. Seems like a minor thing that could have been fixed, considering that NPCs do react to different things in diff ways. Seems like fixing it would have been similar to many other situations the game handles pretty well. My char has been yelled at by a guard for trying to talk to him with her bow out for example. Notable oversight I would say.

This is best explained in the in-game book "Immortal Blood".

You find out that it is very hard to tell a Cyrodillic Vampire from a non-vampire. Cyrodillic Vampires are the most powerful, sophisticated, and most of all unrecognizable. Unlike the Vampires of other provinces, who are somewhat barbaric and easy to spot.

so it's not a problem with my game?

No issue with your game.

Same reason 25% Vampirism works, people don't recognize you as a Vampire. (despite the pale skin, red eye thing).
If you were 100%, though. I chalk this up to minor, accidental oversight.

Lol. A guard I have never met somehow knows I accidentally killed someone on the other side of Cyrodiil and have 'blood on my hands', yet a 300 year old vampire cannot somehow sense one of his own kind. Odd indeed.