To adjust the wiper arm, you pull off the plastic cover on top of the AFM.
If yours is a virgin, you'll have to cut the silicon glue along the edge to
get it off. Once inside, you can move the wheel with the gear teeth
backwards or forwards to richen or lean out the mixture. This wheel tightens
or loosens the coil spring to the wiper arm, and is responsible for the
mixture at higher speeds. At least as I understand it--experts care to chime
in here?

You cannot do this properly without an exhaust gas analyzer. The old days of
tuning a carb by ear are gone. I would have a mechanic who knows Vanagons do
this before you move on in your analysis. It shouldn't cost more than a
half-hour of time. Make sure they tweak the AFM wheel while it's running at
2500-3000 rpm, not just at idle.