AbdulAlhazred
Legend
Claiming to be a Revenant Monk that dables in druid (for the changing into different animal bits) is probably enough to explain most of the stuff (the Rev part pertaining to why the cleric's turn undead hurts him). As long as one party member is in on it and willing to give up the surges (and elp with the lying), they can probably get away with most. Heck the whole biting thing can even be explained as the primal beast form "coming out".
Here's my question though, why would CHARACTERS need to be told anything like that? Do the characters have a list of classes? Maybe they run around with a PHB1-3 and an HotFL and a DDI subscription on their laptop (getting Internet must be a bitch).
In the GAME WORLD there are no classes. There are no defined lists of powers that are packaged together. All the characters have to go on is what they see and what they think they know. So yeah, the vampire might be trying to pretend to be 'not a vampire' and claiming to be "from the monastery over by Bigtown" might be a good cover, or not.
Also, as far as party acceptance, I'd consider that most adventurers seem to have some sort of background that includes not exactly fitting in with the rest of civilization. It is a good chunk of why they're adventuring. A vampire might be a little far out, but he's just another misfit. In a crazy magical world there's not that much of a reason why the party should be rejecting someone who's an oddball.