what dos vitality damage xx% do ?
It functions to increase the amount of vitality damage that a character does by the stated percentage. Vitality damage is mainly inflicted by players using the spirit mastery line. Skills such as drain life do vitality damage. Just think of vitality damage as another type of magic damage with spells, bonuses, and resistances. Vitality damage is a "secondary" form of damage, as are piercing, poison, and stun. You can find your resistence level on the second page of your character screen.
doing vitality dmg is similar to doing piercing dmg. it's just a different type of dmg. If a mob has high piercing dmg resistant, it won't help them again vitality dmg. You won't get hp back from doing vitality dmg unless it is actually written as Life Leach. What Frodo was saying, was spells like Drain Life cause vitality dmg so +vitality% helps those spells. that is, as i understand the effects