Vamptonite is the nickname for human blood that has been infected with the Leviathans' corn syrup mixture meant to dumb all humans down. A side-effect of the mixture is that it is dangerously toxic to vampires, killing them the instant the blood is ingested. Sam and Dean have discovered this and used it as a weapon.
In addition to killing vampires, this weapon also kills werewolves, shapeshifters, ghouls, and all other creatures that prey upon humans, with the exception of leviathans.