I think we need some additional info here. What exactly are you trying to do? There are several different techniques in Vegas to do this - Chroma Keying, Masking, Cookie Cutter FX, etc. Each of which serve a different purpose.
If you have the time to watch, this Sony "Webinar" from a month ago does a great job of introducing many of the techniques and tools in Vegas designed to do exactly what you are asking about:
I had an hour available two days ago, so I just watched it. Gary, as usual, did a great job. The advantage of watching the Webinar "on tape" rather than live is that you can skip ahead through sections that don't apply or where you are already confident you know what you're doing.
I had used all the techniques at one time or other over the past dozen years, but I learned a whole bunch of tips and techniques that will be incredibly useful in the future.