^ I don't think it's really possible for most people to give any sort of informed consent, anyway. I mean, a Scooby has a decent understanding of what it means to be a vampire. The average Joe, when faced with a real vampire for the first time, will probably have to rationalize it as a trick of the light or a gross deformity or just anything other than a real-life fairy tale monster. I think all newbies have to be tricked or forced in some manner, even if it's just by their own brains, except in rare cases (like if one of the Scoobies, a watcher, or a slayer was turned).
EDIT:
I mean, I'm a Buffy fan, but I don't believe in vampires. If I were confronted with a real-life vampire in the Buffyverse model, I would probably rationalize it away as some sort of extreme fan prank or something, you know? I imagine I'd be dead or turned before I managed to wrap my brain around the fact that it was real.