Vitality over what you need for a buster allows for more party mistakes. This is not exclusively -your- mistakes, or the healers, but the dps.

Dps messes up. Healer goes to heal them--you wernt topped off, boss hits you again--now lower than what you would like to be. A healer has to decide who to heal, and might let the dps to die because you are in danger, or who knows the reason.

Keep in mind im not advocating stacking vitality, but that vitality beyond the 'amount needed to survive' only works well in statics where people seldom 'mess up'. So determine vitality based off your group performance.