In reality, the immunocompromised are the real petri dishes for mutations, vaccination or not! https://www.science.org/content/article/uk-variant-puts-spotlight-immunocompromised-patients-role-covid-19-pandemic

Even with near-perfect vaccine coverage, mutations happen and in fast-mutating viruses, they will quickly figure out how to spread despite vaccination (see: flu and covid, for example). Vaccinations (at least in their current forms - future versions promise better protection!) are effective in softening the blow of fast mutating viruses and can significantly reduce the spread and lethality (see covid and flu vaccinations for example), but anti-vaxxers aren't going to affect whether or not the virus will mutate (hint: it will). They'll just help it spread more if they don't die first...