I've been working on my Westy Syncro for a couple of months, with
regards to Dynamat and noise propagation.

First, some notes: If you have a syncro, and your drive shaft joints
are in need of replacing, nothing else you can do will have a greater
impact on noise on the inside of your Vanagon, then getting that drive
shaft dealt with. It creates a LOT of noise (worse in the 43-48 mph
range, but noticable throughout the speed range of your Vanagon.

The Vanagon takes a LOT of dynamat as there is a lot of resonating
material to cover in a Vanagon. In the Front doors, you have the
exterior AND interior surfaces that can resonate.

I used 3 large  sheets of Dynamat and only covered about 1/2 of my
vehicle.

What I have dealt with so far: Both front doors, the sliding door, the
passenger side quarter panel and the rear hatch. Additional I put a
fair amount on top of the engine compartment enclosure (metal portion
only). In addition to the dynamat I also used the "space insulation"
material (aluminum on one side, cloth batting on the other). This was
applied with upholstery adhesive to just about every place I put the
dynamat. Yes, things are quieter, but as my drive shaft needs to be
dealt with on my Westy Syncro, things are not notably quieter yet.

Wind noise is NOT a big issue with my Westy.