I've always wondered what a vampire vampire slayer would be like. I actually wrote about that once in a fanfic. Faith died for like 2 seconds, and the calling was switched around and given to a guy named Jake, but what the scoobies didn't know was that Jake had been bitten and was a vampire also. Then the slayer that was called after him became evil, so Buffy, Faith, and Jake had to stop the other slayer, Violet.
My vampire vampire slayer, since he was also a slayer, could walk in the daylight. I don't know why. That was one screwed up fanfic anyway!