I had a similar problem.  The Vanagon had been sitting for about two or
three years. You said that you bought another one?I guess that may mean is
that you bought a new Vanagon.Correct? Has it been sitting for awhile? Do
you know the maintance history?

I went through all the Bentley checks several times without help for this
problem.My Vanagon was firing on two opposing cylinders.It ran smoothly,
but without power.It was firing so that the engine would idle,too. Like
firing on cyl 1 and 3, or 2 and 4.As opposed(ha ha) to cyl 1 and 2 , or 1
and 4. I thought it may be a cam because the non firing cylinders must
have something in common.

Turned out to be gunked up injectors in  those two cylinders.The other two
wree ok.No fuel at all was getting through.

To fix this,I removed those injectors. I hooked up 12V to the injector, to
open it up. With concentrated fuel injector cleaned (a table spoon full)
in the short rubber hose, I forced this through the injector. I did this
by hooking up a bicycle pump and hose clamp to the rubber hose end. After
three injections of cleaner all was fine.I did the other 3 too.