here is a tip I have used, so you don't want to outlay for a new PFD for the 4 times a year you might use it, just wear an old rashie over your current one.
I found that helps keep everything in place a lot better, helps stop the PFD riding up, and by tucking the rashie under your harness keep you free from tangles.
Added bonus...it makes you look huge. Very intimidating to the other racers