If your bridge footage is high contrast, you could easily create a travelling matte using the mask FX.
If you are creating a Bezier mask in Pan/Crop, place the bridge footage on a higher track so it is in front, and apply the mask to that.
Audio FX works the same as the Video FX. Do you have a more specific question?
Gary Kleiner
Learn Vegas and DVD Architect
http://www.VegasTrainingAndTools.com