I think you might want to look into these.. http://www.wetokole.com/
though it's likely cheaper to buy a set or 2 extra of vanagon seats for the price. 

From:    Joe Federici <joefederici@EARTHLINK.NET>
Subject: water resistant seat covers. . .

I've spent a lot of time cleaning my seats and would like to find
water resistant seat covers for when I go kayaking. The center is
cloth and I know will suck up the water quickly.

I've got an 87 vanagon

Anyone found a brand they feel work and fit well.

JFF