In my experience the most reliable method is to hunt vampires. The only problem is, sometimes vampires blood isn't contagious. The only way to tell is after drawing blood, if the text just says "human blood" - I don't think the disease is communicable - but if it says "human vampire blood" then that means it is, and just drinking a little bit of it should do the trick. So basically what I'm saying is - the easiest way to become a vampire is by being a vampire vampire. They can be very difficult to catch though - I often encounter vampires who will follow me on the fast travel map, but the moment I drop into the regular screen they will flee. A fleeing vampire is almost impossible to catch, as they are ridiculously fast and they never get tired so they can run for as long as they want - which usually they'll run until they're just out of sight, and the moment you walk into range again they'll say "I must withdraw!" - sneaking can help - if you sneak in fast travel mode and then manually drop into the same tile they are in, sometimes the game will place you right next to them. This makes for an easy opportunity to at least make an attempt at a ranged strike - crossbows are a safe bet, but any thrown item will do so long as you can draw blood with it. Sometimes you don't need to kill them, you just need to get them to spill a nice pool of fresh blood. It can be a frustrating endeavor though, and be warned - if you do manage to get them to go hostile on you sometimes they will unleash unholy wrath on you. I've had a vampire I was chasing out of a hearth turn around and kill me in two attacks after I managed one successful blow. I think he punched my face off or something I can't quite remember.