I'm not sure it's the life vest, but there is some droopy piece of fabric on older planes in Y. I don't notice it with the new Y seats, and I don't recall noticing it in F. It doesn't bother me too much.

Actually it seems sturdy enough in its droopiness (if that makes any sense) that occasionally I will use it as a foot rest for a few minutes, just to have some change of posture.