How have I missed a Buffy thread?! 
I'll be singing OMWF for the rest of the day now, thanks everyone...
To turn someone, they have to suck the vampire's blood after the vampire has sucked theirs. Presumably no slayer would ever agree to do it, even to save her life?
They have to suck your blood, then you have to suck their blood. It's like a whole big sucking thing 