I mentioned it, before, but no one else has taken it seriously.
http://www.amazon.com/Bike-Peddler-Cycling-Eyeglass-Mirror/dp/B001VTQNVO
I mount this to the visor on my helmet, but you can also mount it to glasses. This mirror has been manufactured since the 1960s with no changes. No vibration, and has the advantage that, by turning your head slightly, you can "sweep" behind you. It's one of those things that "may take some getting used to, but once you get used to it, you'll wonder how you ever got along without it". After long rides, I've even found myself looking for the mirror while WALKING down the street! I've heard others say the same thing. It's on my helmet, so I never have to worry about it getting broken off of the bike. Blackburn also makes one, which attaches to a helmet with double-sided tape.
I know the reasoning for looking behind, but, are you going to do this constantly? If you do it all the time, then why would a motorist think you're going to make a maneouvre? You've already looked back 5 times without doing anything. I only look back if I'm really going to make a maneouvre. Plus, my long pleasure-rides are done on a recumbent. One CAN'T turn around on a recumbent.
At least consider a helmet mirror. They are cheap enough, that if you find you really don't like it, you're not out much money. I'm willing to bet that you'll never go back to a bike-mounted mirror.