134a systems have "barrier" hoses.

"with the introduction of R134a came "barrier" hose line. Basically a hose within a hose that limits the probability that smaller R134a or even R12 molecules will slowly escape through the hose wall over time (why are you topping off the refrigerant every other year)."

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Before you use Maxifridg, or  R12-a blends  (flammable hydrocarbon blends) you may want to read the info on them at the EPA page as part of your decisions process.

http://www.epa.gov/Ozone/snap/refrigerants/hc12alng.html#q5
http://www.epa.gov/ozone/snap/refrigerants/macssubs.html

Another good article

http://www.autobodypro.com/safety/articles/0031.htm

According to the EPA, R12a blends are illegal for vehicle A/C use.

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Good basic A/C info page
Written with Porsche's in mind, but applicable to the Vanagon, since the A/C set-ups are similar.

http://www.griffiths.com/achelp/achelp1.html