The Vanagon radiator is actually two-in-one design with two cross flow
radiators, one mounted behind the other in a single unit.  The left side
tank connects the two sections to redirect the water flowing out of the
back section into the front section, or out of the front into the back,
depending on how the hoses are connected.  The right side tank is
actually two tanks connecting all the cross flow tubes on the front
section to the outlet and all the cross flow tubes on the back section
to the inlet, or the other way around, depending on how you attach the
hoses.  I don't know whether the front section is connected to the inlet
or the outlet.  There must be an optimum arrangement but I don't know
for sure and I haven't looked.

If the radiator has air in it, the water that enters the inlet tank will
flow across and back to the outlet tank through the tubes up to the
level of the water in the inlet tank.  The reason you need to raise the
engine speed up to 2000 when bleeding out the air is to circulate the
water in the system fast enough so you create a slight pressure in the
radiator to force out the air through the bleed screw even with the cap
off the tank in the engine compartment.