If you vibrate your haed at the same frequency it should make the problem less noticeable. 
Or your head. Either one.
Hmm. Sounds like a GD topic to me. 
For a more serious reply…
SIMS makes all sort of dampening products for archery, music, firearms, sports, etc… Those mini-limbsavers may be just the thing. They attach to nearly any surface with the already applied glue (peel & stick).
I use the larger versions on the limbs of my bow, along with their products on my stabilizer, and string. If they can reduce the vibration in a bow, shooting an arrow, they’re likely just the trick for small vibrations like are induced by the music.
Sure, Lexus could probably damp the stock mirror, but probably only 1 out of 50 of their buyers would even notice, or care. cost v payout doesn’t work.
Great point at the end there…Meanwhile that archery goo looks neat, I’ll hafta research it.
Well, if I’m listening to music with a lot of bass in my car (Chrysler LHS), the rear view mirror vibrates a little. I’ve always just looked at is the price I have to pay for getting a car with excellent factory speakers. It never really bothered me enough to do anything.
Wouldn’t it still vibrate no matter what you do the miorror anyway? Isn’t the windshield actually the thing vibrating slightly due to the soundwaves, and therefore, anything attached to it (mirror) would also vibrate?
Isn’t it just the bass vibrating the air?
There are a couple of different ways the mirror could be vibrating. One is from the stem between the mirror and the mount, and the other is the ball socket where the mirror joins the stem.
To echo brossa, I have seen cars with the mirror attached to the ceiling and have a small ‘pillar’ between the stalk and the windshield. It appears to be made of a rubberized foam and is molded so that it looks like it was designed to be there. I can’t think of the make/model of car that I had seen it on. This would absord some of the vibration being transmitted from the ceiling.
While not the most elegant solution, some automotive sound deadening mat may help. This would allow the mirror to vibrate less since the matting would absord a good deal of the vibration. I would think that you might get away with a small strip on the back of the mirror so it doesn’t look too sloppy. You could probably ask for a couple scraps at a car stereo installation shop (Best Buy?) instead of buying a 12 sq ft mat for a few inches.
The point behind those things I linked to from SIMS are dampeners. They vibrate, but they tend to damp out those vibrations. The same way they reduce the vibrations (and more importantly, the noice) from my archery equipment.
(Note, I am an IT guy, and recreational shooter, I do not work for SIMS, but I really like their products!)