This is a good question and it’s one i’ve had myself over the years. i believe it’s only one strain of vervain that can harm vampires since the only vervain mentioned is the one that grew at the base of the white oak tree— the tree that gave life to the original vampires. the original vampires were created and the white oak tree existed in mystic falls so i assume the only vervain that can harm vampires is the strain that exists/existed in mystic falls. hope that makes sense lol