The best tires for 15-in wheels on a Vanagon will be the tires
recommended for the Eurovan (Caravelle) which comes with 15-in
wheels.  These will be either 205/65-15 or 195/70-15 and will be
"reinforced" or "truck" or "C" just like the Vanagon requirement.
These sizes provide same rolling radius as Vanagon's 185/80-14 and
205/70-14.  Also, perhaps someone can advise tires provided on South
African Vanagons, now available with 15-in wheels.  A good place to
look is http://www.michelin.com/us/eng/news/agilis/3.htm where 195/70
R15C AGILIS 61 TL 100R is listed.