Hum... Maybe I am wrong. I thought I saw from the Bently book that the little
vent hose goes from the neck of the filler to the top of the expasion tank.

The tube that I suspected being too small is the gas filler tube, not the vent
tube, or breather tube. Yes, I should check this vent tube first. It is highly
likely that it is plugged up for some reason.

And, yes the gas filler tube of my 83 westfalia is visibly bigger than that of
my 84 vanagon GL. And it was the original equipment when my friend bought it
in 83. I bought it from the friend 2 years ago. There is a vent tube on this
car too.

I have been wondering if the filler tube, being smaller, causes this choking
headache for my 84 vanagon. The 83 westfalia never had this problem at all. 
Apparently there is no feedback so far of any 84 vanagon which suffers from
exactly the same symptom. I'll check the vent tube which is most likely the
cause.

Thanks Joel.

David