Looking at Provincetown, it's far from obvious that this is what happened. It seems more likely that a lot of vaccinated people got the virus in close proximity to unvaccinated people, but the chain of transmission mostly stopped there. Numbers in Provincetown have fallen and there were no reports of major outbreaks when vaccinated people returned home. So it would seem like you could have vaccinated-to-unvaccinated spread in some cases, but vaxed-to-vaxed is multiples less likely.

With the large majority of the world still unvaccinated, and most spread (apparently) curtailed between vaccinated individuals, the evolutionary pressure on the virus will still be toward longer incubation and less lethality. Perhaps if we ever reach a point of worldwide herd immunity by vaccination, and still have large scale circulation of the virus among the vaccinated, we'd be looking at a different evolutionary drift. But it's also worth noting that the relevant mutations in Covid-19 are on the spike protein, and there aren't so many configurations, only a few hundred; it's likely that all possible configurations have already been "tried", and are currently competing in the wild.