Also most pfds are not snug fitting (let's be honest) so there will be ride up which could/will affect your mobility as it rides up over your head and the arm holes aren't where they should be. The loose fit and ride up, imo, will definitely hamper any movement to exit in a cramped, disoriented vehicle compartment. As Wayne said earlier, it's the disorientation that will make people panic.
I come from a whitewater canoeing background and have been under a canoe in moving water trying to set up a roll so have a small sense of what disorientation can feel like. Keeping calm should be the first thing to do.
Wanted to note as well that the flotation of a pfd shouldn't be underestimated. it won't be a simple matter to just push down and fight the float. These things are meant to keep users afloat so diving down a few feet will be tougher than one would think, imo. This is from experience wearing a whitewater specific vest in moving and still waters.