That's very definitely the dreaded vanagon head gasket problem. The first
time it happened to me, I let the dealership take care of it (Mistake) they
replaced the gaskets and the fix lasted for just about 6 months.  (Gaskets
and things like that aren't warrantied). Six months later, knowing the
symptoms and what was wrong, I took it to a local place (Salim's on Eastern
Avenue in Toronto, I recommend them) and Salim who owns vanagons of his own
suggested that doing the gasket replacement without slightly machining the
head that was leaking was futile, because the aluminum is always slightly
warped at the point of leakage.
That has to have been 10 or more years ago.  I guess he was right. I also
make sure to always have a good mixture of the million dollar blue goo
vanagon coolant in it instead of just normal aluminum friendly stuff, so
that might have had something to do with the prolonged stamina of Salim's
machining suggestion.