What year Vanagon? Is is  a subaru conversion? You ought to be able to tell
if your compressor is working by watching it and turning the switch on and
off. There is probably a low pressure cut off switch that saves the
compressor from burning up when the 134a gets low. In post 86 vanagons,
this is up above the engine area. It has wires attached to it. There is
also a high pressure cut out switch. Dont know 2.1 vanagons(even with a
subie) too well. You can remove the electrical connector from the low
pressure cut off switch9see Bentley) and jump the wires to turn the
compressor back on for a moment.If the compressor now runs, it may be low
on 134a. So quickly? Must have a leak.

 Tell me about your compressor. Junkyard vintage?  If the car was not in
the yard long, and the compressor may look newer, and still have pressure
in it, the compressor may be good. Even new compressors have a shelf life
of 18 months. Then they go back to the mfg. The shaft seal will shrink up
and let the 134a out . Same with an old,unused compressor. It needs to be
used to lubricate the shaft seal.If it sits for more than a year, it is
most likely a bad compressor.No matter how good it looks.