I came across this while browsing, today.  It's a report by Santech with
material compatibility for alternative refrigerants.  I thought I'd
mention it since I've seen these refrigerants mentioned on the list from
time to time.

http://www.santech.com/reference/index.cfm?action=view&pdfid=9

They tested "Freez-12", RB-276 "Freezone", FR-12 "Frig-c", GHG-X4
"Chill-it", "Hot Shot/Kar Kool", and R-406a "McCool".

The results weren't good -- nearly all of them caused some swelling of
most of the seal materials.  Problems included seals swelling to where
they'd no longer fit into the glands, seals splitting, seals extruding
out, seals turning gummy, and hoses collapsing due to softening.

The chart in the PDF might let you pick out a refrigerant that would be
acceptable if you happen to know what type of seals are in your Vanagon.