Never happen.

I know the people on the French Side better than I do those on the Dutch Side. IMHO, knowing just a little bit about vaccine resistance of the French, they're already doing protests in the street over vaccine mandates for healthcare and public service workers, as well as for vaccine passes. A mandate on the entire French Side population will never happen.

Dutch Side, I don't see much difference in acceptance of a mandatory vaccination. Dutch and French residents are not all that different.