I think John is probably correct. In the right hands, and used sparingly, it could in theory be very helpful...but it would also have lots of potential to get people into much more hazardous situations too. There are a few options out there which fit some of your criteria, but I think your final one would eliminate any that I would consider. If you did find something that could be stowed in the back of your fishing vest, I think the set up time to get it ready would make it a bit of a hassle? A good fitting PFD while wet wading has worked well for me in a few situations on smaller streams, but I’d be way too lazy to try that on a wider or faster stream.
"Lord help me to be the person my dog thinks I am"