As a chemical engineer from the pharmaceutical industry, there’s a few issues to explain.
Regarding the virus:
1st - getting vaccinated means if you contract the virus, your body has built up an immunity. It does not mean you can’t spread the virus.
2nd - getting vaccinated isn’t 100% effective. These vaccines, which happen to be the fastest ever developed for a major virus, have effective rates of ~ 75% to 96% so nothing is 100% effective.
3rd - there’s still divergent information on how long a vaccinated person remains immune to contracting Covid, with ranges from 3 to 9 months.
4th - there’s no long term testing done, for both the vaccine or the disease. Because of the double digit mortality rate - the vaccine development was accelerated and inoculation is being expedited.
Regarding masks:
The Covid virus isn't an airborne virus, if it was we all would already have it. It spreads on contact. What makes this virus spread so fast is it's ability to remain active outside the body for a relatively extended period of time.
When humans exhale, moisture is a part of our breath. This moisture is actually very small water droplets. It's the tiny water droplets / moisture in the breath we exhale that contain the virus and spread the disease. A much cruder example is when some sneezes on you.
Wearing a mask helps reduce the moisture in the breath we exhale coming into contact with another person. It's not 100% effective, and I don't believe I'm aware of it being stated otherwise.
So realize getting vaccinated doesn’t mean you shouldn’t wear a mask to help reduce the spread of Covid.
There’s are many more issues, but they’re not for this forum.