Volks,
if the "direct fit" O2 sensor for your 86-91 Vanagon is hard to find or too expensive for your budget you can always get a Bosch generic 3 wire O2 sensor and graft on the wiring connector.

I did this in reverse a while back.

I had a 98 Subaru Outback that needed a new O2 sensor.  Subaru wanted $150 for it.   I took an extra vanagon sensor I had in my parts stash, cut the connectors off, grafted on the Subaru connectors then the Subaru passed smog with flying colors.