The ones that manage to replicate survive. If a vaccine tends to stop some and not others the survivors become the majority. In pest control in orchards, something I have a little experience with, pesticides that are partially effective rapidly become less effective, because of this same thing. Using lower, not very effective rates of pesticides promotes mutations because more mutations survive. If the vaccines were good enough to actually stop almost all transmission they would not allow the virus to mutate around them. But they are not that good. The vaccines do not stop the virus from replicating and from infecting other people.
onlinelibrary.wiley.com
www.canr.msu.edu
Why don't I use examples from covid research instead of pesticide research? Because anybody telling the truth about covid mutating in response to vaccines will be slapped down.