OK, when I posted last time I was out of town and away from my editing PC. Now that I've had a chance to bring up Vegas Movie Studio, here are some options.
(1) Use the pan/crop tool to isolate portions of a clip that overlays another. limited to rectangular masks.
(2) Use the Cookie Cutter (under Video FX) - usually in combination with the pan/crop tool for locating the masked section.
(3) Use the Chroma Key (under Video FX). Difficult to work, but fun to play with. It sometimes helps to use another FX (typically color adjustment) ahead of the Chroma Key.
(4) Use the Deform effect. Strange results, but maybe it'll work.
(5) Use a partially transparent Color Gradient (under Text & Backdrops) to overlay another clip.
These are available on Movie Studio 4. Version 6 may have other options.
Pixelan's SpiceMaster plugins
cost some $$$, but should give you more capability. They're great with Vegas, but I've never used them with Vegas Movie Studio.