Eric,
I'll assume you're in standard def, and that whatever version of Vegas you have works about like my pro version of Vegas.
First, use a paint program to create a 640x480 mask. You could make a 720x480 mask, but you would need to uncheck "maintain aspect ratio" on the graphic in Vegas. Let's use blue for the chroma key color. Most of this mask graphic will be pure blue. But the area you want blocked out will be black (or whatever color you want the blocked-out area to be). Save this graphic as a BMP. Now go into Vegas and import the mask graphic. (You've already imported the videos, right?) Place the mask graphic in its own layer, and move that layer to the top. Now click on the "Track FX" icon for that layer. Select "Sony Chroma Keyer". The chroma key adjustment window pops up. If needed, move the adjustment window so you can see the editor's preview window. You probably won't need to adjust anything, since it probably defaults to blue. If the blocked-out area doesn't have the position and shape you want, use your paint program to change it and save it. You shouldn't need to shut down and reload Vegas. It senses when the graphic has been saved, and will automatically reload it.
Have fun! :)
Ken Hull