A stock Vanagon will run fine on regular gas.  Some of the information on Boston Bob's web site is just plain wrong.

I think its time to pull this out of the archives and dust it off:

Here's the story on octane:  At one time (up til the early '70s?), there
 were two ways to measure octane, research octane (based on lab work with
 the fuel) and another system (I forget the name) based on tests in an
 actual engine.  The two systems usually give somewhat different
 numbers.  Most car makers (including VW) specified research octane, often
 listed as R.O.N. (Research Octane Number?).  The US government, in its
 usual wisdom, decided that since people couldn't agree on a method, they
 should invent a third one!  So, when the government started requiring
 that octane be marked on the gas pump, it was required that the two
 existing methods be averaged to come up with a number (the fine print on
 the sticker even reads (R+M)/2.

 So, when VW calls out 91 R.O.N., as the gas flap sticker on my Jetta and
 the owner's manual for my Vanagon both do, that is the same (more or
 less) as the US gas pump number of 87, which is the usual number for
 regular in most areas.

Bill