Yup.
Same here.
Players are supposed to be player characters....as in use the Players Handbook.
The DM is supposed to be everything else...including, monsters (such as vampires)...as in the Monster Manual.
A vampire is not a player character option found anywhere in the Players Handbook.
If a character ever got bit and turned into a vampire, or contracted lycanthropy and turned into a werewolf, I as DM (the one who is supposed to play those things), would immediately take control of thier character since they are no longer players, they are considered monsters at that point.
Players have tons of cool abilities, feats, and things offered to them without having to move into your territory. If this was a serious campaign, I definitely wouldn't allow it. But if your just coming back to d&d and don't care much about ruining your game with a player vampire on the loose, well sure what the heck, just do it and have fun. Expose him to sunlight or give him a HUGE craving for exotic blood like dragons blood or something, and just learn from this mistake.
It is definitely a mistake.
But it's cool, we all make them, and players naturally want to break the game in cool and interesting ways.
You just gotta decide if you want to play Vampire: The Masquarade or play D&D