So I'm watching a critique of Midnight Sun (Twilight but from Edward's perspective) and the reviewer starts discussing vampire biology (since it's discussed in the book). Carlisle has some speculation but hasn't been able to do proper studies, and the explanation offered doesn't account for waste products (do vampires piss, etc).
My thought: vampires, being dead, are constantly degrading and blood is metabolized to repair their bodies (handwave how that happens); now, we could say that they're outside of thermodynamics and produce no waste from the blood, but let's not, and instead say that venom is synthesized from waste products (leftover elements of blood and non-blood consumables). This kinda works, since venom replaces all their bodily fluids, but it leaves open the question of how it heals wounds.
I suppose we could stretch and say that it operates on a "one man's trash is another's treasure" system (i.e., the remaining "useful bits" contained in venom differs from vampire to vampire). Or we could say that the internal metabolism is picky, but external cells like skin will use anything they can get.
No, I haven't read the books.