Hi Eric, mmmmm.....
Most vanagon i have worked on; low speed kick in at 2/3-3/4, normal temp
(highway) is a bit below or over the light.
The switch and fan system are independente from the rest of the cooling
system, you can easily test the switch in the van or on the bench,
Heat the switch with something (hot air) to test this one and a voltmeter.
But am sure the switch is NOT the problem.

http://www.benplace.com/front_radiator.htm

So the way i see things is the rad is not hot enough!!!
You may have air-pocket somewhere in the system.

Do some more bleeding, find a steep hill and drive this one up and down a
few time, open the rear!!!! and front bleeder screws in between.
You should also check/test the thermostat in hot water.

Cheers, Ben
http://www.benplace.com/