Vienna Stop, Orange Line
From the Vienna Metro, there is a "Metro to Mason" shuttle directly to GMU campus every half hour (see the timing at http://shuttle.gmu.edu/masontometro.html) The bus takes 1/2 hour to get here. The shuttle stop at the Vienna Metro is on the north side of the station - turn left coming out of the turnstiles and take the pedestrian bridge over the highway. The GMU shuttle is at the far left shelter when one comes out to the semi-circle of bus shelters on the north side. (I believe this is the third shelter.) Exit the shuttle at the Sandy Creek Parking Deck stop, a roundabout area next to the parking deck. The driver should be able to tell you when that is.
The math department is on the 4th floor of Exploratory Hall, very close to the Shenandoah Parking Deck.