That's because the pictures are not exactly the same shape.
The black bars will never be seen on any TV, because of overscan.
But, if you want to get rid of them, switch your timeline in Vegas to 720x480 (use 1.21 pixel aspect ratio), then go into pan/crop for your clips, right-click on the frame, and choose "Match output aspect."
Or, you can bring your whole project VEG file onto an NTSC timeline (24p, if necessary) and apply the crop to the whole thing at once.
But I doubt it's worth the effort -- no one will see the black bars.