Not all vests are equal. From an old story on the main ETS site:
As they gathered around the ice chest, the two older men hung on to the handles at the ends. The collar of the vest helped to keep their head out of the water and they could relax somewhat as a result. Jens also was able to relax a bit, but Jim, with the ski vest, had to continually fight to stay upright and to keep his head up. His mouth was about level with the water and if he relaxed at all he would tip either forward or backward into the water, just depending upon where his balance was at the moment. It was tiring.
http://equipped.org/997ditch.htm