MD student here - I think based on the data we have so far and what we know about related vaccines (e.g. influenza, for which very few people wear masks to avoid infection) that the vaccinated individual in your scenario would still receive some degree of protection. I think it’s a fair analogy to picture the vaccine as having a net effect of reducing the exposed viral load - i.e. even if immune cells ultimately can’t fully neutralize a high exposure they should still be able to clear a certain amount of the virus. So if we can assume that exposed viral load corresponds to eventual severity of disease (obvi not a perfect assumption but seems pretty well-supported by existing data), then of course vaccine+mask is more protective than vaccine alone, but even vaccine alone should be more protective than no vaccination at all.
This is probably the reason why - even as capacity limits, etc are being lifted - wearing a mask in general settings where you don’t know who is/isn’t immune will continue to be recommended while COVID is still circulating. Because as you mentioned, all of this very promising data is in the context of mask-wearing and other precautions, and the vaccines might appear to be less effective in the absence of those other actions.