I have not caught up to the six pages after this post, so others may have mentioned the following.
You’ll need to marry you statistics class with other classes. The mitigation may or may not be purely additive. So it is not a pure math problem unless you include many other variables. For example the mask may decrease viral load on host and with the vaccine that brings its effectiveness up. Or it could have overlap and some of mitigation is redundant. I could go on with speculation, but it is not black and white. But I totally understand the use of a simplified model of mask and vaccine interaction. But just wanted to point out that it is a model, not a mathematical or clinical certitude.
... yes, all true. But the general point is simply that mask + vaccine will be better than either one alone. (unlikely that protection is 100% redundant).
Complicating things far more: We are citing the studies for Pfizer and Moderna, largely pre-UK variant, and also not studied against the South Africa variant.
And while the JNJ vaccine looks very promising, it is also notably less effective than Pfizer and Moderna.
So a blanket statement of “vaccines are 90% effective, we don’t need to wear a mask anymore!” Is really a premature statement.