Vinegar reduces surface tension when added to water - hence the windo washing wheeze.
Wasp stings and jellyfish stings can be reduced in intensity if vinegar is applied quickly (Bicarb of Soda for Bee Sting - note the capitalisation as a mnemonic) - it is all to do with vinegar being acidic and the appropriate stings are alkaline. BS/BS is the otherway round I think.