I'll make a tut if you don't get this:
1. Get the image of the piece you want cut out, then make a photoshop file with the same resolution you recorded at. Basically, get a completely black background then cut out the shape you want, however you want. After that, make all of the area you cut out transparent then save the picture...png, or whatever supports transparencies.
2. In Vegas, you can just have two tracks, the bottom one the video, and the top one as the image that you saved earlier. The video should show through the transparent bits.
Let me know if you don't get it, or if it doesn't work .