I don't recall reading that any passengers had life vests on. In fact, I don't recall any vests being recovered as part of the surface debris.
It would seem that the passengers either had no advance warning of the ditching, or they were unable to get to the vests because of g forces or something else ?
If they were in a crash position, at impact, they must have had some amount of advance warning of a ditching.
It seems odd that not one passenger out of the couple of hundred+ would not successfully locate a vest given the ability and time to do so.
It is possible there are some corpses with uninflated vests on in the undersea wreckage but no information of that sort has come to light.