I don't think you could be further off-base than to suggest that either Vincent or Shadow be in Slytherin house. I don't see either of them being particularly ambitious or really very charismatic or diplomatic. As a ninja, Shadow is required to be cunning, but he also requires many characteristics that would make him more suitable to Ravenclaw house. The same is true for Vincent.

Sephiroth (and pretty much every other FF villain) is definitely a Slytherin as are Rufus, Tseng, Hojo, and Corneo's lackey, Scotch.

Reno & Rude would also belong to Ravenclaw house whereas Elena would be more suitable to Hufflepuff. Reeve, despite being the man behind the curtain in the case of Cait Sith, is an entirely different person and would either be a candidate for Ravenclaw or Hufflepuff.

NOW somebody give us the rundown on the house candidacy of the cast of Final Fantasy X.