I am guessing based on the icon that this is something to do with increasing your health but the Nornir chests are also for this so I am wondering whether it is something different. Anyone know what the stat actually does?
I forget how to actually view the sheet where it tells you what all of the different stats in God of War actually do but Vitality does increase the max health. I think it also increases stagger resistance from enemy attacks. Its not the most useful one but it is a handy one to upgrade.
Edit: Here is a breakdown of what all the different stats do in God of War Ragnarok.