As much fun as Vamp!Willow is, I doubt it; that would be a completely different story. They've dropped too many hints about Willow becoming an increasingly powerful - almost god-like - witch; it would make more sense to take that to its logical conclusion rather than turn her into a vampire. (Plus, Vamp!Willow = soulless = probably wouldn't feel sad on Buffy's account.)

Then again, I'm always way off on Season 8, so... I guess she's a vampire. ;-)