code0, whats so hard to understand, as previously stated the VG allow smooth air to flow over the windscreen as opposed to turbulent air. With air swirling around the windows you'll get a lot more noise than if its flowing smoothly over the surface. Just imagine driving along on a very windy day in your car as opposed to on a calm day. When its windy you'll get a lot more noise inside the cabin.
P.S. have you ever heard what a vortex sounds like?