I have been thinking of something similar with Halloween coming up and all that, only no vampires. Heck, I didn't even want to use any typical horror monster, but something from one of the D&D Monster Manuals, just to make it more mysterious. (Yeah, I know many of them are based on real world mythological creatures, but I would go with one of those that aren't, like the Intellect Devourer.
I would say that the biggest key to running a horror adventure, is to keep the monster a mystery for as long as possible. Keep the players (and, thusly the PCs) guessing. If that means you have to give it powers and abilities that are not common to it, then do that.
Here's an idea (I don't usually go with PC vs PC storylines, but for a short run horror it might work great): have one PC be the bad guy behind everything, they have put together the plot to destroy the other PCs - with your help, of course - and it is up to them to keep the plan moving during the game. All the while they are along with the rest of the group pretending to be their friend. You definitely need to have a player that can pull such a thing off, though.