[aerohead]

The original mirrors probably produced wind noise which was deemed unacceptable for the production car.
The VGs can alter the resonant frequency of a structure,moving it up or downscale out of the range of human hearing.

I don't think it has much bearing on mpg.
Acoustic, anechoic wind tunnels allow manufacturers to address these issues of Noise,Vibration,and Harshness (NVH) which is a standard procedure for modern auto design.