Couldn't find this, and it seems stupid to ask.

There is that character Yuri (I think) that can make her self have no max vitality. If during her effect- she gains lets say 5 more than her normal max, what happens after the ability switches off... does she keep it, or does he revert to normal max vit.

You can't have more vit than your max. If your max vit becomes lower than your current vit, your vit will effectively be truncated down to your max.