My guess is the problem isn't really so much the muffler as it is the total lack of sound deadening on your vanagon. Peel & Seal roof flashing (sticky asphalt goo on one side, shiny aluminum on other) comes in 6" x25' rolls at Lowes and Home Despot for about $17. I suggest you apply it to the metal panels on the right rear and rear lid. Remove the trim panel from the rear lid, pull out the fiberglass insulation and apply the stuff to the door lid after you have cleaned off all the dust/junk from the door surfaces. The rear fender area involves removing the upper air inlet grill from the pillar and fishing strips of the peel&seal inside the metal areas. Find a skinny kid to snake his arm down there or use a stick to press it in place. Bang on the panels first to see how much it sounds like a giant empty tin can. Then bang on them after you have applied the peel & seal. Very noticeable difference. Yeah, the dynamat is lighter and better but it's a TON more expensive.

Tom

'79 Mercedes 300SD "Green Hornet"
'84 Westy Vanagon "Spiny Norman"
'86 Alfa Romeo Spider "Dinsdale"
'98 Mercedes E320 wagon "Silver Hornet"