This isn't my fix.
I found it in the archives, and it worked well, and locked them down
permanetly.

Take the mirror arm off the base.
Remove all the hardware.( sping washer etc.) Wrap it in a rag and put
the arm in a vise with the stud up. Lock a Vise Grips onto the hex stud.

Hit the bottom of the Vice Grips with a hammer removing the hex stud.
Drill the stud hole out with the proper fractional drill to accept a 1/4
x 20 tap.
Tap the hole.

Put the mounting hardware back on the arm. Run a 1/4 x 20 bolt into the
base of the mirror mount. Snug the bolt down.
Mount the mirror to the door.
Adjust the mirror to taste.
Remove the mirror and tighten the bolt down.
Install the mirror.

It will never move again.

What I have discoverd with these mirror's is, if you replace a broken
one with any of the list vendors money saving offerings the mirrors will
move and not lock down.
They are cheap in quality.
Mirror's direct from VW do not slide around as easy the inexpensive ones
do.

I posted this to the list for the benift of Mr. Vehring, who was the
first to inquire and complain, but never found the time to inquire.
Pehaps he found a better way to solve  his obvious problem on his own.

Perhaps.

RC