Vitality Die
Term
A 
Vitality Die is the die you roll to determine how much your total Vitality increases each time you gain a level. So the size of the die affects how much Vitality you have, since each level you have a better chance of getting more Vitality. Classes that are supposed to be tough have a larger hit die, and so should have more Vitality.
d100
Mod
ADV/DIS