In short, I wanted to increase every enemy's vitality. But changing the vitality value(as can be seen in the screenshots above) only seem to work until a certain limit. So there seem to be a certain limit.

And after reading both of your posts and the guide from the link above, if I understand it right, the act part is limiting the attributes? In that case, is it possible to increase each enemy's level by 3 or 5 without modifying individual entry?