A friend was recently flying across the channel in a tandem seat motorglider wearing lifejacket, parachute and of course the seat harness. Half way across the bl**dy lifejacked inflated itself. Luckily she could still breathe and was able to puncture it with a ball point pen.
What would you do?
Did she not read the instructions of the vest?
If done properly, the pull cord should remain covered, so it cannot be pulled accidentally. Being such unfamiliar with the vest and its built in valves that she punctured a working vest with a pen does not sound like proper pre-flight too.
So, number one, if the vest is properly operated, it should not inflate accidentally; number two, if it blows, there are mouth pieces to manually inflate as well as deflate the vest to adjust the air fillings to your need. I would do a proper instruction on how to handle a vest as first action.