Sam,

This is a design feature.  My '84 does the same.  The fan starts just 
before I panic.  I believe newer Vanagons have two thermal switches and a 
two speed fan.

I thought about replacing the thermal switch with something of a lower 
value, but I have never gotten around to it.  Our Vanagons have made it for 
13 years that way, perhaps it does more damage to us thinking about it than 
it does to the engine.

Mike