Two points:

1. The idea that you can turn your ac compressor on by adjusting your fresh
air controls in a vanagon just doesn't sound right--at least not in any
vanagon I have seen.

2. The ordinary thermostat that controls the ac operation is not the main
issue. The problem is the special thermostat that prevnets the compressor
from coming on at 38 degrees (according to Joel). This special thermostat I
thought was inside the van but it was mounted directly against the sheet
metal so in effect it reads outside ambient air temp, not inside air temp. I
think the sheet metal against which it is mounted is in the left rear up high
in the vehicle, possibly inside that cavity in the far upper rear where fresh
air comes in. 

For newcomers to this thread: the point is important because you won't be
able to run your ac compressor in winter (and thus protect your ac from
losing its now valuable refrigerant) except on those days when it is warmer
than 38 degrees.  In cold climes, this is a tall order.