There is stigma attached to wearing the vest; for the parents, not so much the child. When you say "autism" to a lot of people, they react as if you've just insulted them or their child. Of course, being the parent of an autistic child, I tend to treat people who think that autism is an insult with about as much care as I would a mosquito -- I swat them away because they're annoying.
That said, "the kid is three, of course he's going to be hyper" is potentially just as wrong as "wear this vest because you're being three and I don't want to deal". If there's a behavioural issue, and the teacher isn't just being a tool, get it checked out sooner rather than later. I know you're likely to get your shoulders up with just the mention of something possibly wrong with your kid, but don't trust that instinct. I almost did and my daughter would never have gotten the help she needed/needs if I had.
Edit: This is not in defense of the teacher's actions. Unless there's a diagnosis, they shouldn't be doing that shit.