In my experience it is not the mirror that vibrates, rather the rear window as the sub is mounted in the rear parcel shelf. If the bass is that loud you have to ask yourself are you listening to the music or working on the assumption that everything sounds best with the maximum bass. Much like the chavs in their Corsas with sub boxes.
Its all to do with sound pressure level. Big speakers (Bass) move a lot of air when the cone moves, modern cars have quite soft seals around the windows to aid in the reduction of noise (some manufacturers name it as a feature) Sound Deadening Glass. The pressure increases in the car as the cone moves forward and the air has to go somewhere so it pushes on the easiest thing to move, streching the seals, moving the glass the mirror is mounted too. High volume external noise will also vibrate the mirror.
Turn the radio down.
Infinity...............
First ever MAX POWER TOP GUN 1998 Best in Show. B)