This article desribes a way to create a single color on B&W , from a color video. It uses the mask check box in the Vegas color corrector. http://www.videoguys.com/Vegas_Tips/TTS01-11.pdf
Is there an easy way to do this in VMS ?
I'm not at my editing computer right now but I seem to remember achieving just that effect with one of the NewBlueFX filters you get with VMS 7 (and 8). I think it was 'colorise' but you have to play about with the settings.