re:  "... extra rubber on the road to cope with rain and
snow and mud and wind ..."

Stock Vanagon sizes are either 185-14 or 205/70-14.  Bentley
manual states chains cannot be installed on 205/70-14
(clearance problems).

Regardless of whether chains are ever installed, LESS rubber
on the road is better for snow traction (more psi on the
ground).  About the only real advantage a Vanagon owner
might get from larger tires is the possibility of more load
rating.  That might be useful once--offset by the fuel
economy penalty of moving bigger, heavier tire every day.