Learn on this page what injuries you need to use vinegar for:

Jellyfish Stings

Often leave whip like sting marks.  Immediately flood the entire stung area with lots of vinegar for at least 30 seconds.  This will stop the stinging cells on the tentacles.  DO NOT use fresh water and do not touch with bare hands to remove.  If pain relief is required, apply a cold pack only after vinegar has been applied.  Narcotics may be needed.