@Boog Greybeards tip for ALL such things. Unvulcanised Butyl Rubber.

A tiny piece shoved in will act like a rubber threadlock. The 3M version is a wee bit hard but pulling and stretching softens it.
Avoid getting TOO MUCH oil and grease on it or it starts to spread a bit.
That or Capt'n Tolleys Creeping Crack Cure.
Or a little rubber solution glue.
 
As used and endorsed by Crackheads everywhere:p:D

s125_captain_tolleys_cure_1.jpg
 
Add a new or an extra spring washer and tighten should do the trick. Or try a few wraps of thin plumbers sealing tape around the tapered shaft as well.
It is loose at the back of the mirror, not the stalk.
 
It actually takes quite a while to dry if not exposed to air as in a ball socket. As long as the mirror holds its position when bike stopped if you did it early evening and then parked it overnight it should be fully cured by morning. Do not over do it though because it dries horribly on exposed metal surfaces with a white crusty look. One or two drops should be heaps.
 
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