I really can't think of any way that the VSS would fail that would cause it to make the speedometer read 80MPH when the car is not moving. The VSS simply outputs pulses (8000 pulses per mile) of 12VDC current--on, off, on, off, etc...
Maybe unplug the VSS and see if the speedometer continues to go crazy, if it does, that eliminates the VSS as being the problem.
