I have installed a mirror and base using 5 minute epoxy to my '91 Passat
windshield.  The mirror is supposed to twist off/on the base.  I did not
want to glue on the base and then try installing the mirror.
So I just glued it as an assembly.  Mark area on the outside of windshield,
sand the glass and base a little, degrease, apply a little 5-minute epoxy to
both the glass and the base, install the mirror and hold it until it sets.
I sometimes use a small light (trouble light) to heat up the windshield to
hasten setting & drying times.
I have even tried the Harbor Freight $0.99 epoxy with good success.  ;-)
I have re-installed many mirrors on Audis and VWs over the decades and
usually I can install them separately securing the base with duct tape until
the epoxy sets.  <g>

I would think that maybe your base will twist off or pop off of the mirror.
Not sure of this though.
Put up a picture somewhere so it can be determined how it gets assembled.
;-)