A general question: what evidence is there that airbag vests help?
A question I would understand is something like "is there a good study on how much an airbag vest helps?".
But especially the kind with cervical stabilization, it's pretty much guaranteed to provide some protection. There will be some situations where the rider is injured instead of dead, or bruised instead of injured.
RCTs aren't science, we're allowed to reason on the basis of readily-apparent empirical facts.
Wow. When is the answer to this question not "Yes"?
Is water wet? Is the grass green? If you're hungry, will eating food relieve hunger?
Are you better off falling off a motorbike wearing a jacket or nothing?
Ok, how about a blow-up cervical collar?
Hint: the current recommendation to place children in the rear seats came about as the result of more than a few children being injured or killed by airbags. It transpires that when you use explosives to solve a problem, the unintended consequences can take a while to work out.