[Shujin]

I remember Dev answering in Lore questions, when I asked him what this "Taboo" was about or if it even gives them essence. The answer was it DOES, but it just absolutely against the ethic of the Vampires, like cannibalism, but it does indeed grant them essence.

Is there any difference between feeding on a vampire and on a mortal?
Not really, though drinking the blood of another vampire is extremely taboo to them. The only exception would be a vampire offering their blood to a starving vampire who could die without it, and even then, it's considered very strange but necessary, and some vampires will just refuse it outright for that reason, even if it results in the other vampire dying.

Also from the Lore questions. Not the one I asked but still kinda answers this here.