You realize that the immunity from the vaccine isn’t instant right? The immunity takes time to build up. At two weeks out from dose 1 you’re only at about 52%. It’s not until 10-14 days after the second dose that your body will have built up the full immunity for that 95% efficacy. So no, masking after you get vaccinated isn’t dumb.
Additionally, we do not yet have data on whether you can transmit the virus after you’ve been vaccinated, since with some vaccines you can prevent disease but not transmission to other people (e.g., pertussis). So no, until we have that data showing it is not transmitted by vaccinated people, asking people to mask up so they don’t get it at all is not dumb.