Vervain, also called vampire's bain, is a plant that is toxic and deadly to vampires.
If brought in contact with the vampire's skin, it will burn them and greatly reduce their healing abilities. If consumed (like say, through human's blood) the will become weak and incapacatated, and their venom will be neutralized. Also, if the vampire has a supernatural ability, that ability will be useless for 3 weeks after consumption. If a vampire is completely submerged in vervain, they will eventually completely dissolve and die. Vervain also weakens all of there other abilities.
Alaric Saltzman, a human vampire hunter, has taken to ingesting vervain regularly with nearly everything he drinks. After locating a vampire, he will give himself a small cut and make the vampire feed off of him, at which point he will kill it after if is incapacated. He has also created bullets coated in vervain, so the bullets not only penetrate the vampire, but creates a burning wound that is slow to heal. He also created vervain grenades, powerful explosives that shoot vervain into the air, which can burn most of the vampire he has targeted