"A few months ago I had my '90 Vanagon's fuel tank replaced ($650)
because
 it had sucked itself into a crumpled and leaky shape.   It just started
doing
it again this week - I can hear the tank going "poing" as it deforms
inward,
and there's a "whoosh" when I open the gas cap...."

yes, there is a venting system on these things, which go over the top of
the tank. if there is blockage in them, it could very easily cause this
kind of problem...mine was just the opposite - one of the vent pipes was
cracked which made for the ultimate overflow valve. scary.