What have you tried at this point, and what didn't work?

The basic premise is to put your green screen footage on a track and apply the Chroma Keyer video FX too it. Adjust the color and the settings to get all of your "green" to turn transparent. Now put your background photo or video on a track below the green screen clip. Everything that was green in your clip should now be transparent and will show whatever is on the track below.

A lot of info can be found in this forum by using the search function (the one just above in the "Forums | Search | Forum Setttings | FAQ" section -- not the one all the way at the top of the page). Also, YouTube has tons of tutorials on most any topic. Search for "vegas green screen tutorial", or something similar.

Here is a basic outline of how to do chromakeying in Vegas, taken from a post I made here about a year and a half ago (and modified slightly here).

1. Put your green screen footage on the timeline. Put your playback cursor somewhere on it so the green screen image is in your preview window.

2. Drag and drop one of the Chroma Keyer video FX presets onto your clip (or apply it by clicking the FX icon on the clip itself, etc.)

3. This is important: In the Video Event FX pop-up window for the Chroma Keyer function, locate the "Chroma Keyer" item in the fx chain at the top of the window and UNCHECK the Chroma Keyer tick box so it is temporarily disabled. You need the effect to be "off" when you are trying to get an eyedropper sample of the green-screen color. (If the Chroma Keyer effect is left on then it has already turned your greenscreen slightly transparent in the preview, so you can't get a good sample of it.)

4. If necessary move the pop-up window out of the way so you can see your preview window.

5. In the Chroma Keyer popup window, click once on the eyedropper icon at the lower left of the "spectrum" color area.

6. Move your mouse pointer to the preview window and now click and drag a rectangular area that contains only the green screen color. You have now shown Vegas a sample of what color it should turn transparent.

Note: New versions of Vegas will not show an actual rectangular outline of your eyedropper sample. (Older versions did.)

7. In the Chroma Keyer popup window, re-tick the Chroma Keyer function in the fx chain at the top to re-enable it.

8. In the Chroma Keyer popup window, put a tick in the "Show Mask Only" item at the lower left of the window. This will make everything you want to "keep" turn white. You might also want to "solo" the video track or make sure there's nothing on the tracks below it so that the "transparent" sections of the mask show as a very black color (rather than showing your eventual background image).

9. Click and drag the "High Threshold" slider down (to the left) until you get a nice, solid white on the sections you want to *keep*. The green screen areas (which are supposed to go transparent) might still be a little milky looking at this point.

10. Click and drag the "Low Threshold" slider to the right until the green screen areas (which should go transparent) lose their milkiness and hopefully turn a very nice black.

11. You might want to add some blur (with the third slider) to smooth out the "chunkiness" of the mask.

12. Un-tick the "Show Mask Only" function, and un-solo the video track or move your background image back under it.

13. Don't expect Industrial Light and Magic results, but by tinkering you can do well enough with most chroma keys. One of the biggest limitations is std def DV footage. It might help to add some small amount of Chroma Blur (video fx) *ahead* of the Chroma Key function in the fx chain. High-def footage tends to look much better because of the higher resolution gives a much less "chunky" masking ability.