its More with less Vit, and Less with higher vit.
so if you have 10 Vit and you add 1, its something like 2,2.5 EE added.
but if you have 100, and add one, its less then 1EE added.
but like Harma said, adding vit adds Hitpoints equal to *stated Vitality **Level * so more vit is always good for a tanker.