This one is mutually exclusive with the cowgirl route, so if you progressed enough in that one, you'll need to start a new game (or reset the cowgirl path). If you go to the pub in Tarkas at night, there should be a girl sitting in the north-east corner. Talking to her will start the vampire path (you'll have to visit back more nights, and then she'll start "visiting" your house when you sleep). There is more to it, but these are the basics.