ACV DOES NOT sterilize water. It is an organic compound that some organisms can readily feed upon, including those organisms in the ACV sediment that are used to inoculate the feed used to yield fermented feed. The rationale behind using ACV should be to introduce beneficial microbes into the water - so that their growth and waste/byproducts overwhelm and suppress the growth of harmful microbes. The effects of the ACV itself will be minimal, but it contains - and will feed - a LOT of microbes, much to the enteric benefit of your chickens. You could achieve the same the same effects with probiotic cultures, but they won't thrive in a sterile solution.
What does everyone think ACV stands for? Apple cider vinegar, right? Well, that's what I initially thought, and that's what I used when I got my first chicks. PASTEURIZED, sterile ACV. I used it for a year before I read "Bragg's, organic, raw apple cider vinegar" in an obscure and long-forgotten (by me) post somewhere in BYC. I concluded that when posters say ACV, what they mean is BRAGG'S ACV. There is a profound difference between the two: one is a probiotic inoculant, and the other is a sterile, mildly acidic solution used in salad dressings.
Using Bragg's in water is an iffy proposition: if the water is clean, it won't support the growth of the inoculant, and if you add nutrients, you may not be able to selectively control what grows in it. What I prefer to do is keep my water as clean as possible and introduce probiotics by way of fermented feed. This is as simple as adding a tiny bit of Bragg's to some milk and pouring it over scratch to cover - and letting the whole mess stand overnight in a warm place. The Scratch absorbs the inoculated milk, then ferments and grows the probiotic microbes. my chickens love it. Not messy to feed, either, as a supplement.
For keeping water clean, I have read on some BYC posts that people are having very good results with Oxine AH, and I've ordered some to try it out. I use LARGE water vessels with nipples, and it would be nice if a bacteriostat helped to keep the water potable between fillings. Blessings