I personally would not do the vinegar. Clean, fresh water is really the best thing for her. Her digestion is probably slowing, so that may be causing most of her issues, as well as not absorbing nutrients as well. If you want to give probiotics I would use an all species one, like probios, that you can mix with a bit of moistened feed, or sav-a-chick makes one that mixes in the water (make sure it's the probiotic and not the electrolytes).