Vincent could have Geostigma and just not show it because he's really hard, unlike Cloud...wussy! Nah, only joking, Cloud isn't a wuss...well he doesnt look like one in AC at any rate. Vincent could have Geostigma, easily, remember we havent seen that much of him in the trailers, so he could have, it just hasnt been mentioned yet.

As for Cloud, Seph and Vincent undergoing transformations .etc. Well, arent the J-cells pretty unpredictable as to what they actually do? Its effects could simply differ from individual to individual. Hojo's experimentation on Vincent looks to be pretty slap-dash in the game. I mean isnt he lying on a desk??? Not surprising his limit break transformations were so screwed up looking, - he wasnt carefully planned like Seph, and to a lesser extent Cloud.