I noticed while playing that the amount of Vitality absorbed from monsters will change over the course of the game.
From what I have observed so far, there seems to be a boost in VIT absorbed some time in Chapter 3; she gets 0 VIT from the Golem in Misty Cave in Ch1-2, but it seems that from Ch3 on, she gets 30 VIT from it.
When I did some testing on the Ancient Dragon in Derkarr Gorge, I noticed that I got 710 VIT for defeating it on my post-game save file, while I got 370 VIT from my Chapter 4 (True Path) save file. Maybe there's another boost during post-game? Or VIT earned scales to max. stats?
I'll do some testing, but if anyone else finds anything, please comment.