You may be better off trying to blank out the screen instead of changing it. Either way, what you have to do is create a graphic, put it on the track above your video, and keyframe it. It will take a little work to get it to track the camer move (OK, maybe a lot of work). I did this with a pennant on the wall of a gym. I exported another pennant into Photoshop, made everything except the pennant transparent, changed the text on the pennant, put it on the track above my video in Vegas, and then every 3-5 frames created a keyframe and moved and zoomed the pennant so that it exactly overlaid the pennant that I was replacing (temporarily setting the opacity to 50% on the pennant overlay makes it easy to get the overlay exactly where you want.

Try to use the fewest keyframes possible. Otherwise it becomes extremely difficult to get smooth motion between the keyframes, because Vegas gives you no direct control over the spatial and time interactions between keframes other than the placement of the keyframe on the timeline, the smoothness control, and the LInear, Slow, Fast settings for each keyframe. These are crude and not very direct, and therefore you cannot easily create moves that accelerate or move through space exactly as you might wish. However, if your camera moves are smooth, using just a few keyframes, you should be able to do what you want.