I think your talking about when they make a circle around the area of intrest and the rest of the screen is much darker. This is very easy to do in Vegas... Create two tracks, the original clip should be below a copy of that clip only the top one should be darker, (use the brightness and contrast filer). Then use the cookie cutter filter and place it on the dark clip on top which should show through to the original clip below. This is known as masking only the mask is at about 50% opacity.