I still don't know how Vic could be human in the mirror universe. Unless my head canon idea is true in that he is actually what Felix looks like. Felix being the guy who gave Bashir lots of his holo-programs. Perhaps he based VIc's looks on himself. In the mirror universe he even goes by VIc as in a nickname or alias.
The guy in the Mirror universe is only called Fontaine, so what makes sense to me is that the real guy is Felix Fontaine in both universes.