Could the vampire-hunting game have been "Hunter: The Reckoning"? (from all accounts, a serious love-it-or-hate-it title) That's more zombie-riffic than vampire-oriented, but I imagine you'd remember it if you'd been playing the "Buffy the Vampire Slayer" game.
Consider "Deathrow", a futuristic sports game that's sort of a cross between soccer, ultimate frisbee and Rollerball. It's a sleeper that's full of action and is a good multiplayer title, from what I've heard. Apart from that, you know my opinion on the Xbox -- there's not much I'd want to play on it PERIOD, let alone in the multiplayer genre.
