Its not black and white - we're not going for 100% security. Masks never did that anyway, and it wasn't the point in wearing them. The point was to change 'R0' to less than 1, so the epidemic wouldn't grow geometrically and swamp the healthcare system. It's quite reasonable to say that vaccinated people are less likely to spread the disease than say, an at-risk person wearing a mask. If those two are commensurate, th…
You see all people around you wearing masks, you wear a mask, these simple facts act as reminder that it is a pandemy, that you have to be careful about what you do (keep distance where possible, etc.) besides wearing the mask in itself.
Soon you will see lots of other poeople around you not wearing masks and you have no way to know if they are actually vaccinated (and probably in this case you are "safe") or if they are simply not wearing the mask because they ignore the recommendation or don't believe in masks or whatever (but they are not actually vaccinated so potentially "unsafe" if they come near you).