As Dennis noted the Radiator fan has two control points. One is if the
coolant system is too warm (hot), the other is if the AC system is too
hot.

The AC system in your Vanagon is 3 major components. The compressor,
the evaporator and the condensor. The condensor is that section in
front of your coolant radiator.

The AC system works by taking a refrigerant (R12 in stock AC systems),
evaporating it (since it takes heat to evaporating a liquid, this cools
the surrounding materials). Now you have a low pressure gas. That has
to be put back into a liquid form. That's what the compressor does.
Compressing the gas also makes it hot. You have to get rid of that heat
somewhere, so it is piped to the front of the vehicle where there is
hopefully some air flow. The condensor removes some of the heat, which
causes the refrigerant to lose a little pressure and get cooler, where
in it returns to the evaporator to repeat the cycle.

In your AC system you have a relay behind the rear most drivers side
pillar. This relay checks the pressure of the refrigerant line on the
high side. If it is too high, then the AC system is either overcharged,
or you aren't getting enough cooling up front. No matter what the cause
the relay will turn off the compressor and turn on the radiator fan to
high speed until the pressure in the high side comes down to acceptable
limits.

At highway speeds you might not even realize this is happening until
you suddenly realize your AC system isn't giving you cooled air
anymore.

Note there is a low side AC relay too. This makes sure that there is a
minimum charge of refrigerant in the AC system before it will allow the
compressor to kick in.