Leo et al --

I worked in a glass shop for several years, and we routinely replaced side
view mirrors as you wish to do.  It is only a second-best solution, as it will
never be as good as the original, but it's cheap.
You need to break all of the old mirror out of the plastic frame and glue (we
used clear silicone caulk/adhesive) a replacement in.  The difficult part is
making a good fit.  In the glass shop we had a variety of glass-cutting tools,
including a water-cooled belt sander type of grinder.  Generally after much
trial and error, we were able to make a decent looking replacement.  Use 1/8"
mirror.
Eventually we started to make plexiglas templates for the various "common"
applications (can't remember ever doing a vw)  If I remember correctly, we
only charged around $15.
Sould you try this at home, be sure to use clear silicone.  The various
colored silicones contain components that will eventually "eat" the silver
backing off the glass

Were I you, I'd just have the local glass shop handle it.  As in the recent
discussion re: windshields, if it breaks, they've got to replace it, not you.

LOL
Doug