IIRC, there is a phillips (sp?) head screw in the bottom of the mirror assembly, and if you remove it the glass should come out. Once you get the glass out check and make sure all the screws are tight. That should take of it.
Here's what I've done; get some styrofoam (non-crumbly type) and cut some shims to fit around the mirror. That will hold the shaking to a minimum until u get around to fixing same. I've been using this for a few years - yea, lazy.
Be careful in tightening the phillips screw on the bottom. Its purpose is to secure the tri-point mounting system inside. Over tightening will break the plastic threads inside.
I had the same issues, I took out the lower screw, took out the mirror and put a few dabs of silicone in the inner frame/mirror support, reinstalled mirror. Mirror stopped vibrating and is fully functional. I had added the silicone as tightening the screw did not help much.