Belt vibration can also be a source. Check drive belt, if too loose it will flap around and make the thing shake.
I notice a while back that the right mirror vibrates less than the left, only difference is the right has a dampner, the brake resivor holds fluid that dampens the vibration a little. So with that in mind why not fill handle bars with oil, need some type of seal on both ends of course. Another thought was to simply get a tube of RTV silicone and inject it in the bars. they will be completely filled with silicone. Not tried this but wonder if it would do some good.