The acid in the vinegar can react with some metals, same reason you dont use aluminium pans etc for wine making - unless you have the luxury of stainless steel drinkers :-)
There are an untold amount of poultry "pick me ups" on the market, Vinegar being just one of them, others are poultry spice (Verm-X have just launched their version of it), Tonic with seaweed, Rooster booster, fulvic acid mixes - the list is endless......
My personal vies is: if you feed a good quality food and allow free ranging then all you would need from time to time would be a tonic, such as Poultry Spice. Although some of our customers swear by regular use od cider vinegar in the water.
Thanks