Vital Energy (VE) / Vitality is mainly used in rituals, the amount used depending on the vitality bar set. It is also used in healing creatures where you use some of it to increase the vitality of your creatures.
Creatures have their own VE, which can be combined with a players VE to overcome their normal limitations.
Regenerating and Increasing your VE
There are several ways to increase your VE. One way is to wait for regeneration cycle to end, which will grant you 500. Other methods are Free Credits, certain spells like Heal and Give Vital, or moving in your homeland if you have a citizenship.