What you want to do is put 3 spots that are at about the 50% (some will say 70% zone which may be better for vibration purposes) zone of the mirror (2.5" radius) that are large enough that when you squish them to the thickness of two quarters, the spots will be about a quarter or a little larger in diameter.
lay down 3 stacks of quarters (two quarters to a stack) and place the mirror on top. Leave the assembly to dry for a few days so that the silicone is cured all the way through. You can then remove the quarters and you are done.
Good luck and have fun looking up!