Acv certainly has it's benefits, but it's not recommended during times of high heat. It also makes calcium a little harder to digest in your laying hens. And remember not to add to metal water dishes.
http://www.the-chicken-chick.com/2012/04/beat-heat-how-to-help-chickens-survive.html
http://www.the-chicken-chick.com/2012/04/beat-heat-how-to-help-chickens-survive.html