Yes, I had this problem and also solved it using washers (I think they are M20). The vibration didn't seem to get any worse, but the directness of the steering was a revalation after the bendy feeling the soft rubbers produced. Oddly, off-road was not a problem; in fact, when rocks deflect the front wheel on rocky tracks it was better.
The rubber mounting on the mirrors eventually disintegrates in a similar way to the bar mounts. I got by this by pouring the thin silicon winscreen sealant in - this is thin enough to consolidate the rubber. I suspect pouring in one of the really thin epoxy (cross-linked polymer type)could solve bothe the bar mounts and the mirrors, but I haven't tried it. Good luck!
P.S., Ich sprech nur Saarlaendisch (Deitsch!)....
Tall men see everyone else's bald patch but their own.......