Apple cider vinegar is different from ordinary malt or wine vinegar, and also for chickens its different from the over-refined culinary version you can buy in supermarkets. If you want to use it you need t get it from an agricultural store. Beware, though - it may help to tone up the chickens gut, but is totally ineffective at killing and controlling worms. For this you need to dose them with Flubenvet, every 6 months. Also, be very careful not to use too much ACV in their water - they really dont like the taste, and actually many experienced poultry keepers no longer use it because the birds tend to only drink the minimum, which is not good for them.
There are many poultry vitamins and tonics available, some for adding to water such as Lifeguard, some come as powder or granules to add to feed. If you want to give ACV or other liquid tonics, another way to do it is to mix a drop or two into the water you use to create a damp crumbly mash as a treat in winter. This is more economical as if you add it to the drinker most of it gets wasted when you change the water. Also they get to drink plain water in their drinker, which they prefer.