Vitae is the life force that animates Immortals. For Vampires, “The blood is the life”. For Angels it may be divine light.
Each character has up to 5000 Vitae. Your Vitae level is displayed at the bottom of the HUD and the Assess command will display the Vitae of other Immortals around you.
If your Vitae reaches 0, you will die. Fortunately, death is but a minor inconvenience for Immortals and you will quickly rise up as a Revenant.
Revenants of Undead species cannot Feed and must be Offered Vitae, be resurrected by a sufficiently powerful Vitae Gem containing over 3500 Vitae, or another Immortal with the ability to resurrect revenants.
Revenants of Breathers (living species) can also be resurrected by a sufficiently powerful Vitae Gem, or may be offered Vitae by the Undead, however accepting Vitae from a Contagious Undead species (e.g. a Vampire) means that their curse will be transferred to you and you will be turned to their species, or a hybrid of one of your current species plus their species. You choose at turning time.
Living Breathers (with Vitae above 0) can accept Vitae from Undead species without being turned, but doing so will result in you forming a Blood Bond with them and will move you to their group, along with anyone under you in your previous group.
Allowing the Undead to feed upon your Vitae will decrease both your Virtue and that of the Undead who feeds upon you.