That scenario only plays out if vaccinated people can be carriers. If not, virus that is exposed to the vaccine will not have a chance to evolve more quickly.
Even if they cant, for the period when there are lots of people vaccinated and also lots of people infected, that's lots of mutations happening in infected people and then trying to jump into vaccinated people, giving it more chances that one of those mutations would then spread from vaccinations people even if current mutations don't (which isn't known yet). If it spreads from vaccinated people already then that would be even worse.