Based off of what, the assumption that it arose in an unvaccinated person? The working theory I've seen is that it evolved all of its mutations within a single immunocompromised individual. This would clearly be possible, vaccinated or not. The lion's share of the mutations exist in the spike protein antigen. If the individual had been vaccinated, the greatest selective pressure on the virus would've been on that antigen, given that all but the attenuated virus vaccines produce an immune response to that single antigen.

I'm vaccinated, I think that the vaccines are effective at certain things within certain windows of time, but their epidemiological role being laid out as binary is highly overstated and oversimplified.