Well, yes you can, and it is done with the exsisting plugin Fx that came with Vegas. It is the Chroma Key Fx. However this comes with a few caveats.

1 - For the best results the "background" has to be either solid blue or green. At least the background must not contain any of the colours present in the "foreground".

2 - The lighting of the backgound must be sufficient enough to make a solid colour.

Having said all this - I've had some brilliant results in using Chroma keying and adjusting the "sensitivity" of the Key-out background colour to more or less remove the background. In fact it can produce, with diligent application, some rather interesting and superb, almost "paintily" results. I've used a "blue" sky, with a few wisps of clouds, to act as the chromered-out track - works for me. But this is more about my own personal style and might not be good enough for your requirements. There are other, more comlex ways, whereby you can alter the chorma keying track's colour to be more "sensitive" to the chromed out colour IN the background.

But in any case, all this is possible within V4.

Have fun,

Grazie