I didn't read all the back and forth on this thread so I may be repeating something that has already been posted.
The vibration comes from a insufficient support for the mirror. If you can quiet down the vibration of the support you will control the vibration of the mirror. For example you could put a sheet of rubber or rubber washers under the mounting bolts. Or you could glue a heavy material to the backside of the vibrating sheet metal. For example Dynamat or similar.