How to change gradually from color to black and white in sony vegas 8?

Could anyone tell me how to change a clip in Sony Vegas 8 gradually from color to black and white?|||I don't use Sony Vegas products, but it would be key framing. There should be some kind of filter you apply to your footage to desaturate it (turn it black and white). You should, in theory, be able to key frame that filter so it goes from 100% saturation to 0% saturation over time.