It assumes the person knows how to straighten themselves out once they get the reminder. With most people, this isn't the case. Because being reminded doesn't work to do the straightening out, this vest could be so annoying that nobody would wear it for long enough to really change themselves - it takes more than three weeks of constantly being reminded to change postural habits, even if you do know how to do something that really works. Think of how many people disable their the buzzer that is meant to remind them to buckle up your seatbelt.
If someone really wants to improve their balance, move easier and look better, they can study Alexander Technique and it will work. It costs more and takes awhile to learn, but your freedom to move will indefinitely improve over time.
Once you see this, you'd probably design a collar that measures at the occipital joint where the head joins the neck that buzzes if the person's head drops...which for most people would be all the time!
I think that people don't really want an absolute measurement of how bad their posture is - they want to be able to move out of being stuck when they want to.
reward: I want Alexander Technique to be taught in American high schools