> the vast majority of viral replications take place in susceptible infected bodies, i.e. predominantly those of unvaccinated individuals.
I'm not sure this is true, but even if it is, this...
> variants, therefore, mostly arise among unvaccinated.
...does not follow from it. The key thing driving evolution of variants is not number of replications but selective pressure. Selective pressure is much larger in a vaccinated individual than an unvaccinated individual (or at least an unvaccinated individual that has not had COVID and does not have natural immunity).