Is the mirror loose (as when you touch it) or does it just vibrate? If it's vibrating you can try sticking some self adhesive wheel weights on the back side of the mirror. This may change the resonance frequency of the mirror enough to get it to stop. If it makes it worse, either add more weight, or take them off and suffer (i. e. call the dealer)
If it's loose, you may be able to take the mirror apart and find a loose screw or something else, my 97 had a screw that never quite got tightened all the way, someone's torque screwdriver must have been off that day. .