I don't know if higher strength vinegar has been tested, 20% strength vinegar is sometimes used as a 'green' herbicide, though you have to use gloves and take other precautions against acid burns, and it isn't systemic, most of the time it kills off the portion of the plant you spray it on but it won't affect the roots, so the weed often regrows. And it isn't selective, whatever you spray it on gets affected.