I don’t think it will ever be *mandatory*
It will be highly promoted, may say you have to wear a mask if unvaccinated, They may not hire if unvaccinated. But its doubtful they will actually fire employees for not taking it. Especially now that the recent study from Cleveland Clinic states that immunity from having the illness is as effective or better than the vaccine.
I suspect demonstration of sufficient antibody titer will be accepted in lieu of the vaccine