Vitae, in vampire terminology, is blood. The blood is what sustains the undead body, although it doesn't produce it by itself, thus the reason vampires must take the blood of others.
Vitae is the primal essence and energy of the Cainites, and also, their source of immeasurable pleasure. The amount of Blood a vampire can absorb into his body, as well as the power he can drain from it in short notice, is determined by the vampire's generation. However, one thing is true to them all: their undead bodies use a little amount of Blood each and every night to animate itself - when the Blood becomes scarce, a vampire becomes more inhumane and more prone to Frenzy, giving way to their inner Beast.
A vampire wouldn't die if starved of Blood. Should a vampire run completely out of Vitae in his system by any means, it would fall in a state of mental and physical inactivity known as Torpor. The blood of animals is not as nourishing as the blood of humans. Though an animal may physically have a greater volume of blood than a man, vampires draw less sustenance from it. Old blood is never as nourishing as fresh blood. In fact, many vampires refuse to drink old blood, whether it comes from human corpses, blood banks, or a vampire’s private reserve.
The blood of other vampires, particularly Elders, is quite potent. Essentially, elders have greater blood pools not because they are bodily larger than younger vampires, but because the blood they ingest is more concentrated in their ancient veins. Werewolf blood is similarly potent.
Apart from nourishment and pleasure, the Blood has other uses to Cainites:
- It is the fuel of a vampire's Disciplines.
- Spending blood is the only way a vampire can heal wounds (for just as their immortality prevents the Kindred from aging and dying naturally, it also inhibits the recuperative processes natural to a living body). Likewise, rising from the state of Torpor voluntarily is only possible through the expenditure of blood.
- A vampire may use his or her mastery over the blood to increase his or her physical attributes (although this ability is hindered by the Cainite's Generation).
- When the vampiric blood is consumed repeatedly by an individual, it has the property to create a supernatural link of fidelity and dependency towards the vampire who shared it, thus forming something called the Blood Bond.
- A vampire may gift a mortal or animal with a dose of his or her Vitae, allowing the mortal in question to inject or ingest it. For so long as the mortal retains the Cainite vitae in his or her system, she or he is considered a ghoul.
- Using his or her Blood, a vampire may will himself or herself to appear more human for some time: flushing his or her skin, drawing breath, even becoming capable of engaging in sexual intercourse which, while helpful in certain types of feeding, does not mean that the vampire may inseminate a mortal or become pregnant due to being clinically dead. However, it should be noted that only vampires who cultivate their Humanity may use Blood in this manner (the more inhumane the vampire is, the more arduous this process becomes; Vampires on a Path of Enlightenment have forsaken their human sides entirely, making this task all but impossible).
- Last but not least, a Cainite can use his or her Blood to create new vampires through a process called the Embrace.
Vampire: The Masquerade 20th Anniversary Edition, p. 268-270