Okay, been busy doing my homework and I'm going to stick with the niacin ACV mix for this reason.
Apple cider vinegar is fermented juice from crushed apples. Like apple juice, it probably contains some pectin; vitamins B1, B2, and B6; biotin; folic acid; niacin; pantothenic acid; and vitamin C. It also contains small amounts of the minerals sodium, phosphorous, potassium, calcium, iron, and magnesium. Apple cider vinegar can also contain significant quantities of acetic acid and citric acid. It is used to make medicine.
I think the vitamin benefits of the organic vinegar I use will definitely do more good than harm. The ducklings seem to have improved a little more. I've been periodically dipping their beaks, and they have now drank almost 1/3 of their water. The one that was the worst just drank twice and is moving and stretching his neck around while standing up. First time today I have seen him standing and not stumbling backwards.