On Oct 9, 2023, Don Tremblay in Rutland, MA, USA wrote:
"I tried the Halgreaves or Thackary washers and they did not reduce the vibration. Personally, I attribute the vibration to the manufactured attachment method of securing the stem to the base, that being swaged. Over time, the mechanical junction is compromised and this results in a pendulum effect causing a visual vibration.

A few years ago, I decided to restore a couple of original MGA Wingard Mirrors and my fix was to drill out where the stem meets the base and then drill and tap for a countersunk Allen head screw. Doing this also provided me with the opportunity to have the original stems and bases re-chromed. Once restored, I added hardware store sourced rubber grommets that I adapted to go through the dash fixing screws and contained within the confines of the base recess such that the grommets were slightly thicker than the recess of the base so when I tightened up the mirror base to the scuttle, the grommets came in contact before the base did. No vibration". -- Don