One of my pet subjects, so I just have to get my 2 cents in :-)

Method #1:
The vehicle manufacturer specifies one or more tire sizes with
appropriate load ratings.  They also specify inflation pressure for each
size.  The size of the contact patch does not vary from tire to tire as
long as the inflation pressure remains constant.  If you use the vehicle
manufacturer's specified inflation pressure, you will always have the
same size tread contact patch on the pavement.  The shape of it may vary
slightly, but the amount of rubber in contact with the road remains the
same.  This method only works if you are using a factory specified tire
size.  It does not take into account the effects of loading 1500 pound
of cargo (humans, beer, etc.) to the van.

Method #2:
The tire manufacturer designs the tire with an optimum sized contact
patch.  When the tire is inflated to the maximum rated pressure,
applying the maximum rated load to the tire will produce the correct
contact patch.  Weigh your Vanagon at all 4 corners, and calculate the
percentage of the maximum tire load that is being applied on each
individual tire.  Inflate the tire to the same percentage of the maximum
  inflation pressure.  The relationship between load, pressure, and
contact patch size is not linear, but this will get you very close to
the right pressure.  Compensating for additional cargo is as easy as
weighing and recalculating tire pressure.

Method #3:
Prior to buying new tires, take the van to a competent alignment shop.
Bring your Bentley if the factory alignment specs mean anything to you.
  Double check the numbers and never trust the specifications in the
shop's manual.  Buy a tread depth gauge.  Measure the depth of the tread
  when the tire is new.  Take frequent depth measurements at each edge
and in the middle of the tread.  Rotate tires often.  Front and rear
tires wear very differently on Vanagons.  Though there are more
variables involved than inflation pressure, tires wear more in the
center when they are overinflated and more at the edges when they are
underinflated.  Adjust pressure to compensate for uneven tread wear.

Use either method 1 or 2 according to your own concept of how tires are
supposed to work.  Try them both to compare the results.  If you are
completely obsessed with optimum tire performance, use method 3 to fine
tune the pressure.  Increasing tire pressure based on a good guess at
how much weight you add to the van for camping trips is probably better
than doing nothing.  It's overloaded/underinflated tires that blow out
on the road.  For what it's worth, I use method #2 alone.  Using the
weight of an empty passenger van, calculated pressures are very close to
the factory recommended pressures on the door jamb sticker.

Mike Snow