"Probably not an issue if it is changed frequently."
But then, if you're changing water frequently, you're probably throwing away most of the vitamins because of the amount you need to put into the water in order to make sure each chicken is getting a daily vitamin dose.
I'd skip on putting Polyvisol into the waterer and just give 3 drops a day per chicken, depending what illness your treating.
If you decide to use the waterer method in order to dose all birds, I'd get water soluable poultry vitamins from the feed store. I haven't used it, but I think it's called Avian Charge? It's got minerals & vitamins and is meant for the waterer.