re:
> I'm wondering whether there are different hose diameters used in
> different model years of Vanagons.

if you mean in the heater circuit ...only two sizes pop into my
mind..the 5/8ths you mention..
and the next size smaller T-ing off to the rear heater- could be 1.2 inch.

If you mean in the entire cooling system of various water-cooled vanagons ..
VW loves to have lots of hose sizes ...and even special hoses that are
one size on one end, another size on the other end.
To many variations to count.  Even shaped and molded hoses with T's in
them.  Wait until you get a Diesel Vanagon someday, and you'll have even
more thousands of questions then !  ( just teasing, Jack )

Hope it's not 2 long or frivolous for some members.