This isn't like antibiotics. The 66% is how many people are provided some level of immunity, just reducing the infectible population; it doesn't directly cause issues for the virus the way an antibiotic interferes with the functioning of a bacterium. It might culture a strain that's better at infecting folks for whom the vaccine doesn't prove effective, if there's a common factor there and the virus can use it.
It is not yet known if any of the vaccines completely prevent infection; there are concerns the vaccinated might still be able to be asymptomatic carriers.
Yeah I guess I was asking if having only partial-immunity effectively acts as a filter or creates pressure so that rarer mutated strains make it through and are given a pathway to infection/reproduction.
From your response: "It might culture a strain that's better at infecting folks for whom the vaccine doesn't prove effective, if there's a common factor there and the virus can use it." it sounds like the answer is yes.
It might make it through, but in that case it would be better adapted for those the vaccine didn't work on; this could actually make it less infectious for those the vaccine would work on, so it's questionable whether it would be any better or worse overall.
It is not yet known if any of the vaccines completely prevent infection; there are concerns the vaccinated might still be able to be asymptomatic carriers.