Ah, I misunderstood your original question a bit. If the vaccinated were still acting as carriers, the critical mass would still be important (though I'd imagin

Ah, I misunderstood your original question a bit. If the vaccinated were still acting as carriers, the critical mass would still be important (though I'd imagine the math would be different) because the window of transmissibility would be much shorter and drive R way down. Vaccine would mean less viral load, less viral load means less transmission overall. This seems less likely to me than what is actually happening (vaccine inhibits transmission), but I could picture mechanisms by which that would happen.