Hmm, I wonder.......

Other recent GM vehicles have the mirror attached to a metal slide mounting on the windscreen. Check yours, if it has a spring clip at the bottom you can press this (screwdriver may be useful) and slide it off the mounting.

Then stick a bit of cardboard packing in the thing and slide it back on. No more vibration. It may take a couple of attempts to work out how thick the card needs to be to wedge it firmly while still allowing it to be fully slid down until the clip engages.

I had exactly the same problem on a different GM vehicle after the screen was changed and that's how I fixed it.

a couple of months on warranty left although prob not covered!!