I've had similar frustrations with helmet & eyeglass mirrors. I recently started using a mirror from
Rearview Gear. The designer is an engineer and a cyclist. This thing stays put without much vibration and the optical quality of the glass is great. It's the only helmet mirror that hasn't annoyed the snot out of me. The arm is made froma spoke, and it mounts with a very strong magnet. Once you get it tweaked it doesn't change adjustment on you. The only complaint I have is that because of the magnetic attachment, if I physically hit the mirror when my helmet is off, I tend to knock it off onto the ground a lot. The black coating has peeled off the arm because of this, but it still maintains the adjustment.
Right now they are doing garage production and only selling them on their web site.