*shrugs* Well, it's just a hypothesis. It just seems to me that there must be something IN the blood which keeps vampires going, and lifeforce is as good an explanation as any. (It would also explain how sometimes the victims become vampires themselves - presumably, if vampires can extract lifeforce, they can also inject a bit of their own, perverted lifeforce into the victim in the process, which remains in the body after death and resurrects them.) Still, it's not like I can provide documentation to support my point or anything.