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Vitality > 20 gives more health / health per level
Willpower > 20 gives more energy / energy per level
I've noticed that equipping some +Wilpower items on my mage (with > 20 willpower) doesn't add to her Energy pool. I think I've noticed the same thing for vitality. I'm wondering if increasing these stats over 20 with skills will do anything in the long run.
Attribute bonuses are not applied retroactively. Vitality and Will only affect how many hit points or energy you gain when you level up. Each two points over ten gives you one extra hit point or energy each time you level, on top of a small random amount.