Most (I think) air cooled Vanagons had auxiliary, gasoline
powered heaters installed.  In any event, if you live where
winter occurs, you want one.  This is something important to
verify is operational when buying a Vanagon.  It has its own
electric fan (one speed) and connects into the normal heat
circuit so there is defrost.  (But compare the size of the
defroster vents to the area of the windshield.)  It's been a
few years now since my '81's been out of town, but one ski
trip I remember I think I pretty much was never able to do
without the auxiliary heater.  Vanagons have lots of windows
and effectively uninsulated metal surface area and you sit
really in front and your feet ...  Theory has it this heater
can be run without the engine running, but I've never tried
it because it uses lots of electrical power.

Without the aux heater working, engine heat is available via
engine cooling fan with fan speed varying with engine
speed.  I don't think any Vanagons came without that.  It's
only ever effective on warm days or possibly if you could
drive at engine rpm redline.