Actually, the rear fog light is intended for the space which
is used for brake light in North American Vanagons, and the
empty place is intended to be the brakelight (or may be
switched with what is now tail light--I think this is it).
This would explain the "smaller bulb" notion.

If factory wiring for rear foglights exists, it's likely to
be gray/white.  This is according to my old "VW Air-cooled
Vanagon" manual by Haynes, which includes a "Supplementary
wiring diagram for Transporter and derivatives--rear
foglamp--UK vehicles.."  I think clues here indicate it's
unlikely factory wiring exists on the early Vanagons at
least, since it appears to indicate a separate fuse adapter.

The statement that fogs only work with headlights is born
out by the fact this diagram indicates power to the fog
light switch is obtained by tapping into the black/white
wire coming from the light switch that's only powered when
it's in headlight position.  Since this diagram indicates
this, it appears switching to high beams does not turn off
the rear fogs, which I find mildly surprising since most
German automotive electrical systems do cancel front fogs
when this occurs.  (This is intuitively obvious and very
"German" I think, because high beams would be unusable in
fog, and fogs would be ineffective when high beams are on.)
For these older Vanagons, I think tapping power from either
fuse 3 or fuse 4 for the foglight switch is the way to go,
and this will automatically provide the high-beam cut out.

I asked the "Canadian" question a couple months back, and a
reply to this list indicated there were no rear fog lights
standard there either.