Ron and others are correct that phospate-free coolant is
widely and less expensively available.  I believe most will
be labeled DEX-COOL compliant, which refers to a new GM
specification (they invented the DEXRON word too).  I think
that the specification itself calls for not mixing it with
other types, and if a specific brand says is's compliant
with the spec but doesn't mention the no-mix requirement,
then that's for "marketing convenience."  The no-mix
requirement is for the extended life feature.  You can read
some about this orange stuff at ...
http://www.texaco.com/tlc/products/havololant.htm
or
http://www.prestone.com/cooling/extended.html

Also, while it's clear the dealer is competing with other
sources for most consumables, I'd be really surprised to
find any dealer that feels it should compete with, say,
Wal-Mart on price.  Probably the senior citizen greeter
won't be able to assure you a particular product is ok for a
Vanagon either.

Finally, it makes no sense to think any automotive
manufacturer would design a cooling system unlike any
other.  Can you see the big ho-hum that would greet the
advertising campaign?  Phosphate-free is a pretty standard
Europen requirement--BMW and Volvo being others with this
specification. With the Vanagon, VW may have designed a
system that's unique in how often it fails, but that's
another story oft told at this site.