The virus mutates in everyone.

The mutation rate is generally similar across strains.

Infections that last longer tend to accumulate more host-specific mutations.

The immunocompromised, especially those who have cyclical immune function/suppression, have been documented as the source of variants that accumulate multiple beneficial mutations over the course of infection.. moreso than those with a functional immune system.

The vaccinated, especially those who are disinhibited / stop taking other precautions, continue to spread variants that will be highly likely to eventually escape vaccine immunity.

The unvaccinated provide a reservoir; I'm not convinced they're the major drivers of more virulent variants' evolution.

So do animals, both domesticated and wild.

Polio and smallpox are vastly different.. apples and oranges / not a useful comparison.

Vaccines with zero other preventative measures mean the virus will eventually escape vaccine immunity. It's not the problem, people are.