I use 2 tbs of ACV in 2 cups of water as a rinse after co-washing. Then I rinse with plain water. I'm not an ACV expert, but I believe it lowers the pH of the hair (a good thing). It makes my hair soft.
Ohh! Such fond memories! My mom used to pour about a cup full of ACV on my hair (non diluted), then rinse it when I was a kid. I did not have a perm. I am not exactly sure why she did this, but I do know that ACV is an anticeptic, so it might have helped to clear the scalp of unwanted bacteria or something. Who knows, my scalp was always so fresh and so clean afterward!