Hello SLF,
This is the professional Vegas forum, you really want the Movie Studio forum, but since the procedure is the same...or very similar:
1. Shoot your subject against a background that is a solid color. If you're shooting digital, green is best.
2. Open that footage in VMS after capturing. Put it on the Overlay track
3. From your FX tab, drag up the Chromakey tool. Turn it off by unchecking the checkbox at the top left.
4. Use the eyedropper to select the solid background color. Turn the Chromakey tool back on by checking the checkbox in the upper left corner.
5. Put your new background media on the Video track.
6. Adjust the parameters of the Chromakey tool until your foreground/keyed subject is clean against the background media. You might find it easier to check the "show Mask Only" checkbox while adjusting these parameters. You want a solid white mask seen in the box. When you're done, uncheck the "Show Mask Only" box, and you should see your image clean against the background.
Lighting for Chromakey is much more difficult than using the tool itself. Spend a little time learning to light, and you'll be much happier.