Wow, 12"? I have a feeling that is part of your problem, the thing is sticking out there so far that the stem is vibrating in sync to the bike and causing the vibration. I personally use the Mirrycle ( example: http://www.amazon.com/Mirrycle-MTB-M...bicycle+mirror ). I imagine you can get it at any LBS or they can order it for you. It is a lot shorter than what you request but is remarkably stable. It is also convex but not as much as cheaper mirrors I've used. I also use it BELOW the bar - that is instead of sticking UP I invert it to drop DOWN. This keeps it out of the way if I have to flip the bike, not as handy but it works. Perhaps it would help your situation.