On my ST the left mirror seems to vibrate far more than the right mirror...
I think you might have luck solving the problem if you remove the mirror from the motorcycle, carefully inspect how the mirror mates to the little sub-frame under the front Tupperware, and then re-install the mirror.
There are quite a few little bits that need to be properly lined up in order to allow the flat face of the mirror mounting bracket to fully and snugly connect with the metal sub-frame. Part of the plastic front fairing (Tupperware) fits in between the mirror and the sub-frame - in other words, the mirror mounting bracket and the mirror ittself serves to attach and firmly hold several parts of the front plastic fairing to the sub-frame.
If you are getting excessive vibration from one mirror, my guess is that the mirror mounting bracket is not making a perfectly tight mating against the sub-frame. You can't fix this simply by cranking down on the bolts, you have to make sure everything fits snugly before you even put the two bolts that hold the mirror in place into their embedded nuts in the sub-frame.
Another possible cause, though less probable, is that the mirror itself might be loose where it connects to the mirror mounting bracket. Try adjusting the mirror - it should be a little difficult to move and put up a bit of resistance when you try to adjust it (compare it to the other side). Although unlikely, this could be the cause of your problem... but I'll bet my money on improper alignment (fitment) of the base of the mirror mount to the fairing & cowl stay.
Lastly, be aware that the three little prongs on the mirror cover fit into snap-fasteners bolted onto the fairing. These little snap-fasteners have petals around the edge, and also a very small ring around the petals to keep them snug against the three posts on the mirror cover. Over the years, some of the petals can get broken off or deformed, and some of the little rings can get lost. Either of those problems will result in a loose mirror cover, and if the mirror itself rests against a part of the cover (as it does on my ST when I have the mirrors adjusted as I like them), that will cause vibration of the mirror. The little snap-fasteners are cheap and can be easily replaced - if any are damaged or missing the little round rings, order and install replacements.
Michael
