Stock sizes for Vanagons (Transporters) are 185-14
(185/80-14) and 205/70-14--both have effectively the same
rolling diameter.

If going to 15-in wheels, appropriate sizes (i.e. same
rolling diameter) are either 205/65-15 or 195/70-14, as used
on Eurovan (Caravelle) and, I suspect, on SA Caravelles if
they come with 15-in wheels.

Note:  Bentley manual says cannot use chains on 205/70-14.

See the tire size comparison calculator at
http://www.dsm.org/tools/tiresize.htm

BTW, 185-14 or 205/70-14 or 205/65-15 seems to have been a
favorite with Germans as it was the size used for many years
on S-class Mercedes and 7-series BMWs as well as our
Vanagons/Eurovans.