There are several ways to do this. One way could be:
In a photo editor, you can create a mask. This is a black and white .jpg picture with the size/resolution of your video project. The picture should be completely black in the region where you want to see only the background video and white at the area where you want to see the rolling edits. (Alternatively, you can make this in Vegas with the media generator tool).
In Vegas:
Create three video tracks
Add the black and white mask to the upper track
Place the rolling text one at the middle track
Add place the background video on the lower track
Add the mask generator fx to the mask event at the upper track and choose the option LUMINANCE
Choose for the composition mode of the upper track MULTIPLY (MASK)
Make the middle track a child of the upper track
Now you will see at the area of the white spot of the mask the rolling text and at the area of the black spot of the mask the background video only. (Ofcourse you will not see the mask itself…)
Similar results can be obtained with the cookie cutter fx.
Theo