I happened to have an 84 Vanagon that does not have any cooling system problems
and an 83 westy that does. Here is my suggestion based on my experience.

Go get a stock thermostat. It costs about $32 - $35. It's expensive but can
save your engine so it's a fair deal.

I found that the thermostat on my 83 westy leaky. When it leaked when the 
engine was cold, it took almost forever for the engine to warm up. It was
fairly easy for me to say the thermostat was leaky. The VW dealer which
serviced it drilled 2 little holes on the thermostat. The previous owner,
who happened to be a buddy of mine, turned the westy in because of overheat
problem. Since then the engine seemed never warm up any more.

With the thermostat replaced, $35 gone, the engine warmed up as fast as it
should. I even suspected the temp is too high now.

If a new thermostat does not work you can return it to the dealer. They have
been making too much $$$ from all of us anyway. Good luck

David K.