I recommend that you read the Steve Wright book which has a whole bunch of tips and tricks for dealing with real world keying problems.
Try to level the luminance of the footage so that the screen appears flat but the FG has a heavy vingetta instead. it can be done like this:
CODE
FOOTAGE = ColorWheel(720, 576, 2, 0, 1, 1, 1);
RGrad1 = RGrad(720, 576, 2, width/2, height/2, 1.06609809, min(width,height)/4,
min(width,height)/4, 0.5, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0);
Brightness1 = Brightness(FOOTAGE, 1);
Mask(Brightness1, RGrad1, "A", 100, 0, 1, 1);