I don't think there's an easy way the reduce wind noise in general, unless
you're talking about whistling noises due window seals or from fresh air
intakes. When I remove the dash to replace the heater blower I put FatMat
sound deadener on the nose panel, I reseal all the fresh air hoses. I also
put FatMat on the door panels and all other side panels too. It quiet down
quite a lot, say about 50% but wind noise is still there. Keep in mind that
a vanagon is not a luxury sedan/SUV or modern van. I am a classical
guitarist and I am very sensitive to noises/sounds and the such.