Pax life vests not MEL?
Out of DUB a couple of days ago, I noted a life vest on the floor in front of me, got an FA's attention, explained where it was found, handed it to her. She went off with it as if it were an empty coffee cup to be disposed of. And that was the last of it.
Assuming it was indeed off the seat in front of me, in the unlikely event we ditched enroute to London the person seated there was needing to be an excellent swimmer. So I'm surprised they didn't get someone to check and re-attach it before departure.