What is Venom? [ven-uhm] – noun 1. the poisonous fluid that some animals, as certain snakes and spiders, secrete and introduce into the bodies of their victims by biting, stinging, etc. 2. something resembling or suggesting poison in its effect; spite; malice: the venom of jealousy. 3. Archaic . poison in general.
Or maybe not.
Venom is a
plot device ( plot device? Twilight has plot devices?) fluid that essentially fills vampire bodies like a water balloon, because that's exactly how venom functions in an even slightly plausible species. It is responsible for absolutely everything that happens in their bodies, including lubricating their crystaline skin so that it can bend (demonstrating the world renowned flexibility of solid crystal.) It also lubricates their eye sockets, because humans might find the grating sound of a stone eyeball against a stone skull every time a vampire moves it's gaze oddly disturbing. As an added bonus, it will flow into the penis of a male vampire under no pressure and against the force of gravity, allowing vampire sex (no, really).
Venom is acidic enough to melt contact lenses, but fear not: it's perfectly safe to put into your mouth/uterus anyway. Unlike the venom of every other venomous creature in existence, it involves no enzymes or poisons, but rather carries some sort of bizarre retrovirus that implants extra chromosomes into the victim's cells. This also allows it to somehow fertilise an egg. When bitten by a vampire, the venom is exceeding painful, and causes you to make idiotic sounding comparisons between broken limbs and pleasant hours floating in a pool (or maybe that's just Bella). Eventually, if the vampire who bit you does not drain your blood completely, you will become a vampire, in the same way that the victim of a snakebite will eventually become a snake. Don't argue, it's logic!
Of note, the White Wolf Vampire: the Masqurade roleplaying game gave a similar explanation, with the only fluid inside their version of the vampire being blood envenomed by the vampire curse. That means their eyes were, officially, continually leaking blood. The games manual specifically said "Try not to think about this. It's magic, and it's not supposed to be scientifically workable." Stephine Meyer however remains firm that her world is not magical and does not have these little suspension of disbelief handwaves. Make of that what you will.