There are two factors to consider when trying to add materials to
your Vanagon to make it quieter in the interior.

One is quieting the sounds created by the metal panels of the vehicle
(B-Quiet and other such items deal with this). The other is absorbing
sounds being generated OUTSIDE the vehicle (wind and tire noise being
the prime issues here.)

B-quiet and similar products dampen the "ping" sound of metal panels.
I.E. the quiet the drum like capabilities of these metal surfaces.
Usually they don't do much else to absorb other sounds. You need
additional materials to quiet the other sounds. Mineral wool is an
excellent sound quieting material. Don't confuse it with fiberglass,
which mostly doesn't quiet anything.

The problem is, putting many of these other sound dampening materials
into the panels of your Van can have other detrimental effects.
Namely absorbtion of water and the consequent rust caused by the
proximity of water to metal. To deal with water, you have to put
vapor barriers in place as well as a way to allow water to drain, if
it does pool behind your panels.

Different tires can make a substantial difference to highway road
noise as well. After all, you are sitting right on top of one of
them, and there's not a lot between you and it that is absorbing any
road noise.

VW in later years added a rubber strip along the leading edge of the
front doors to combat wind noise. It makes quite a bit of a
difference, but it is not particularly easy to retrofit to earlier
models as you have to remove the door to install it.