Well, according to Joss in BtVS, vampires came about because one of the last true demons who walked the earth bit a human and shared its blood before leaving. A bit of a sore loser I guess...Anyway, the demon blood turned the dead guy into a lesser demon who fed on human blood and when he bit someone else who then sucked his blood they also became a vampire. Who then sucked on another who then sucked on another and so on. As Buffy would say, it's a whole big sucking thing. :[
This is just one version though. There seems to be lots of different interpretations concerning vampires. Personally, I love the 'hopping corpse'
jiangshi Chinese Vampires who are basically dead bodies being transported to their graves by a Tao priest in a kind of Spiritual low budget type way 'cause the grieving families are very poor and can't afford a wagon.
I'm also very fond of the Legend of the
Yuki-onna, a type of Japanese female vampire who thrives in the Winter and sucks a man's life force or body heat rather than blood.
Wikipedia has an absolute ton of info on vampires and Dracula that makes downright fascinating reading...
Here's a linky if you're interested -
http://en.wikipedia.org/wiki/Vampire