Vitality is a weak stat to raise, considering that it's a controlling attribute for 0 skills. Also, it's retroactive, meaning that points put into it late will count just as much as points put into it earlier (unlike games like Morrowind, where early Endurance is key).
The HP progression (with each stack being about 20 HPs more than the next at endgame) is:
Mage 1
Psionic 2
Bishop 3
Alchemist 4
Priest 5
Gadgeteer 6
Rogue and Bard 7
Ranger, Ninja, Monk 8
Samurai 9
Lord and Valkyrie 10
>
>
Fighter 13

Meaning that the Fighter will have 3 stacks of HP more than his/her closest companions, the Lord and Valkyrie (note that the Fighter levels faster, widening the gap).
This means that Vitality benefits the Fighter the most, who doesn't need the HPs (especially since Ascension Peak enemies will rarely get into melee range), and casters the least, who really need the HPs (again, if you can make it to AP, physical damage won't be an issue).
Iron Skin can be more than made up for by the Alchemist's Body of Stone spell, which grants around 70% damage resistance.