The Vanagon is exactly wrong for aero-efficiency...Actually, if it were a
perfect square rather than a rectangle, it would be worse, but..

  So, some of the things that could be done for better slippery...

  A front splitter would be about the easiest thing of all.  A splitter is
just a 'shelf', a flat projection off the front, parallel to the ground.  In
essense a splitter fools the air into thinking the vehicle is much longer by
stacking up air on the top of the splitter.  A splitter would also reduce
the airflow under the van.  Since we probably wouldn't be looking for
'downforce', limiting the flow under the van would reduce the turbulence
there somewhat.
  Flush windows would be a help also, and do-able.  However, the square
front corners of the windscreen probably starts turbulence there right there
that continues for "a mile" behind the van, so back to the "Entry" the
nose...the splitter..  Raking back the front windshield would be rather
effective also, but beyond most of us, what with the doors right there, etc.
  Smooth wheel covers would be an easy improvement..Like those old Moon
hubcaps..Mirrors that didn't stick way out would help some also.
  But, given the shape of a van, the job of  significantly improving it's
aerodynamic coefficient of drag would be....daunting..
  Don Hanson