Acoording this video, I can tell it's the loose bracket. I think you need to secure the connection of the bracket, as @11811 said.Video of the vibration without stiffening the mount.
EDIT: The view in the mirror was much worse than the recording. Hard to tell which lanes the cars were in.
I don't seem to have any significant vibration issues. I will say that when first mounting the wolfbox custom mount I did have some vibration but found quickly that the mount was not completely seated to the windshield tab causing the the set screw to come loose. It also has so many wires attached to the the top of the mirror I press it up until it hits the plastic trim above and this seems to give the mirror some extra support.
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