If you have a team of 4 people with +70% Vitality and +2 AP per round, I'm pretty sure the rest of the game will be far to easy. You can probably kill enemies who are several levels above you with ease..
Hard to say, certainly the last fight would be easy but it is easy if you are at level 8 anyway. That is not the point.
It is tricky for me to say as I've played this act probably more than 50 times. What happens in later stages I've no idea. Perhaps they will be hard.
Vitality isn't terribly interesting at the moment - either you are dead or not. You are better off spending on armour than vitality.
AP however is extremely useful - if you have a crossbow and 6 AP per turn then you win. Add Flesh Sacrifice and Adrenaline you have 3 shots.
