Joel, thanks for so much detailed info regarding the problem. I looked at my
Bently manual last night at the fuel supply section. I tehn realized that it
has a very complicated setup for being a California vanagon.

The vent tube actually does not go into the gas tank. Instead it goes to a fuel
expansion tank (one on each side of the gas tank, behind front wheels). There
are at least 4 small tubes connected to each expansion tanks. I think the
purpose of the expansion tanks is to allow air/fuel vapor to escape, when
filling gas tank, into them. From there the air/fuel vapor can be sucked 
through a charcoal filter into engine intake. Thus no toxic vapor is escaped
into the air.

However, When filling tank from nearly empty to nearly full, there is a lot
of air/vapor to be expelled out of the gas tank. The two expansion tanks have
a volume of about 2 gallons? I can not imagine how it would not choke, 
especialy filling quickly. I have not allowed any full service gas station 
people to fill gas for me since the first time it was "served". The poor guy
started filling and walked away. At the moment he stepped his first step away
the gas spilled all over and scared the guy to death. He yelled at me asking
if the tank was full. Well, shouldn't have tried to earn extra money in the name
of full service and actually trying to attend other customers at the same time.

Anyway, I'll check the vent tube the first thing next weekend. It do make sense
if it is plugged up. Again, I still have the question, does anyone have any
difficulty like me to fill up gas tank? Mine is a 84 Vanagon, a California
breed.  Thanks for the feedbacks.

David