Here's all I know about Vgon fresh air:
(1) the black dust on your dash is what's left of the seals on the edges of
the doors that shut off fresh air when you want to be warm.
(2) in winter you turn off fresh air intake by duct-taping over the duct on
the front of the Vgon, behind the grille.
(3) your aircooled Vgon is s'posed to have panels like cardboard soaked in
asphalt, closing off all of the grill area except the fresh air intake. 
Replacing these if they are gone is a good way to get MORE fresh air, for
times when you want to be cool (ie when you have taken the duct tape off the
fresh air intake).
(4) your aircooled Vgon will put more heat in the cabin if you let the
air out somewhere.  This is one of the nice things about a sunroof -- you
open it even in the winter.
(5) the levers for heat and cool operate doors by pulling AND PUSHING on
stiff wire cables.  If the wires bend, the pushing part won't work, and
if the levers aren't tight on the cables neither pushing nor pulling will
work, so look inside the dash if something isn't doing what the owner's
guide says it should.
(6) one of the few differences between 80 and 81 Vgons (besides that 81s
have the black removeable "gill" louvers for intake air) is that 81s have
a fresh air blower; 80s rely on air speed to get fresh air in, so the
integrity of the panels around the air intake (see above) is even more
important.

B.
Wm Kennedy
kennedy@admin.njit.edu