Vinegar helps the dye set better and with bolder colors. <3 It's the recommended treatment for nylon fabric, so it works quite well on nylon hair. As for pony bodies it might work, it would depend on whats in the plastic. To use vinegar put the object in the hot dye bath and wait (30 secs? for hair, 5 mins for pony?) then dd like a tablespoon of vinegar.