One more thing to check....when you set the AFM statically at idle, you are
adjusting the air/fuel curve throughout the engines rpm range.  I would
monitor the O2 sensor output under load at WOT (wide open throttle) and
make sure you see .8V or better.  As you probably already know, the O2
sensor is not accurate beyond 14.7:1 air fuel ratio, and varies
significantly with operating temperature.  However, the reading of .8 or
above will tell you that you are not running excessively lean under load.
Your engine will be very unhappy with sustained lean operation.  If you
really want to monitor accurately you will need an air/fuel ratio meter
combined with a wide band O2 sensor.  These will very accurately measure
air fuel ratios from 10:1 to 20:1.  I'm not sure exactly on the vanagon,
but the ECU probably goes open loop above 3500rpm, you should see the
voltage go from oscillating around .45V to a steady .8V or better.

I'm using this LCD device to monitor...

http://www.lascarelectronics.com/PRODUCTS.CFM?STOCKNO=EMA%
201710&CFID=904178&CFTOKEN=88389902

You may actually need to set your AFM a little rich at idle and let the ECU
lean the mixture for you in closed loop operation.