Vitality Dice are used to increase your maximum Vitality when you level, and at other times, such as when you take the Heartiness Keystone Feature.

You also roll your Vitality Dice at the end of a Rest to recover Vitality. Your maximum amount of uses of your Vitality Dice is equal to your character level. When you complete a Rest, you regain 1 use of your Vitality Dice.