Well, this debate has gone just about as far as it can...there's an extremely, extremely slim chance Vincent was a Sephiroth clone and Zack survived(Read:No and no), which could be used for fan fiction, but, yeah. If we were to continue, it'd go like this:
Me:"So, Zack was shot repeatedly in the face with a machine gun but survived because of his Jenova healing powers, and was also a Sephiroth clone for some reason. Also, Vincent was a Sephiroth clone who headed towards the Reunion but didn't make it then went back(And apparently re-locked the door and put the key back in the safe with the monster)."
You:"Hey, it could've happened!"
...Wait, I just realized how insane your entire theory sounds. No, there is no way that man is Zack. If it was, why would he have the number tattoo? Cloud didn't get one, now did he?
I guess this debate ISN'T over, so
<Image edited by Kishi, for the sake of the children.>