Need help with a multilayer zoom in Sony Vegas 9! Please! I have no idea what to do!?

In an animation I have a bacground that is 720 by 480. For part of it I have a peice of stockfootage of flames. To make the flames fit into the bacground, I have to stretch the size of it. In part of the vid, the background, and animation, zooms in. How can I make the flame footage zoom correctly so it stays in the same part of the background (A fireplace) and then dissapear off screen as the camera zooms in further and it should no longer be visible?