According to the Revan novel, Vitiate is half-human/half-sith before the ritual. After the ritual, the book basically says he is now his own race as he evolved to the point that he no longer falls into a current racial category.
Hmmm, yes this seems most likely. The Revan description is vague, but this makes sense. After all Vitiate was the illegitimate child of a common farmer, assumedly human - so Vitiate would have been a hybrid.