Vaccine doesn't make people immune, as far as I know, rather is supposed to lessen the severity and mortality, which it seems to do in some cases. The vaccine simply prepares the body to deal with covid when the person catches it--recovering from covid is what makes people immune. So every unvaccinated person catching it and recovering contributes to herd immunity, while every fully vaxxed person still living in mortal fear and locked away in order to not get infected, is not contributing to herd immunity.