Great idea, but I really don't have any vibration so far- at least that is uncomfortably noticeable. I have gone about 34mph on my bike now and can't really tell I have a motor on it. I posted my idea a while ago, and it got very little response. It's been nothing but successful for me since day one. And it cost me about $5.00 to make. The big fear was the stress on the back mount. So far I haven't noticed any breakage or stress fractures at all. I used rubber in all area's, and that seems to absorb most of the residual vibration.
I would be curious to see what you come up with though.
couple pic's of what I did:
Since the pic, I added a neoprene washer on top of the bolt for any backlash from the spring.
carrot! excellent thinking and ingenuity! please make a build thread about what parts you used and where you got them, i think your idea is the best solution i've seen so far in respect to mounting issues.