If you find lice or mites, it would be best to get some permethrin garden dust or 10% spray from your feed store. Lice need treating at 10 day intervals, and the coop also would need spraying after bedding is cleaned out. Do not treat unless you see lice or mites. Vinegar will not treat them, and would be harmful to spray on a chicken full strength.
All of the questions asked by Biophilliac are good ones to give us an idea of what might be wrong. Crop problems, internal laying, parasites, and respiratory infections can all make hens feel ill.