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!
