Has anyone tried this ?

Schwinn Handlebar Mirror

I find that it vibrates a lot and gets constantly out of alignment making it thoroughly useless.
The mechanism to lock the mirror in position is a plastic threaded collar and I am afraid to strip the threads by overtightening.

I have another mirror - no name - and it works fine when mounted on a bar end but on a bike without bar end the arm is too short to be effective as rearview mirror. Is there any mirror available with about 12" long arm that can be mounted on a handlebar on the right side of the left hand brake lever clamp?

I also find that convex mirrors are quite useless as when the car in the rear is visible, it is too close.

I do have a Blackburn helmet mirror but have not yet managed to find a good position to fix it to my helmet.

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: https://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.

best mirror out there for non-road bikes: https://www.amazon.com/Mirrycle-MTB-M...cmu_pg__header

12" is my desired length for a mirror that clamps on the bar next to the brake lever clamp. The Schwinn mirror vibrates because it is loosely (after making it as tight as one can) attached with a Velcro strap around the grip.

I have a pair of these on my bike. It's a bit hard to get onto regular sized mtb bars due to an odd clamp design but it can be done. They also do not vibrate very much and have a very clear viewing pane (at least for me).

https://www.amazon.com/Sunlite-Deluxe...ds=bike+mirror

It has 9" long arm and 11.5" length, may be what I was looking for.
Thanks

For the record, I've used the Mirrycle Mountain mirror on drop bars for quite awhile. Just mount the mirror directly to the bar insert. Wrist and forearm are in the way at times, but it's a way to use a good, stable mirror.

Lately I have bought way more bad mirrors than good. On my Northroad bars the best mirror is a marriage of an Oberlauda mount and stem with the mirror of a Third Eye. Nice flat mirror that does not vibrate but it's too damn expensive to make another. I recently got an Electra that is working well. The mount on it is very sturdy, the mirror is flat and it has a long enough stem so that my arm does not block my view.