I see this post is what, a month old? But as no one else said anything...
Most people seem to thing that "green screening" is some sort of atomic operation. You have your footage. You apply a chrome key technique. Presto! Perfect footage with transparency where the green screen was! Now, you can just slap any ol' picture you have laying around on your computer behind it!! Doesn't matter if said picture has different perspective, lighting, grading, environment, etc! (But I digress) Of course, our footage better have no green in it except your green screen, and the lighting should be perfect, and the edge of the green screen should be very defined, and you had better hope there is no color spill, or shadows on your green screen, etc, etc. My point? Chroma keying is hard, and applying the chrome key process is only a tiny fraction of the process.
Regarding your situation. This is an easy chroma keying job (that duplo actually worked quite nicely), you just have know that applying the chroma key operation is just part of job of green screening. In this case, and most cases in general, the most important complement to the green screening operation is using masks. Masks are quite simple in concept. A mask is basically a grey-scale image the same size as your footage. The mask is used as a control on where operations are applied in an image. In the case of green screening, you would take your footage, take a green screen operation, first control it with a mask, and then apply it to your footage. Where the mask is black the green screen operation will not affect your footage. Where it is white, the green screen will affect your footage. I have no idea about Vegas Studio, I took to Google for a minute and did not find much about being able to use masks (which can go by different names, mattes being the most common), so sorry if this sounds harsh, but, if a chroma keying software does not have masks, ditch it (for chroma keying). So basically, you would want to mask off everything in the frame except the area around the small green screen. Then apply your green screening operation. Another tip. Masks can generally be used to control transparency also. Depending on how much of the green screen gets covered throughout the clip or whether the camera moves, you could simply use a single mask to set the screen area transparent.
If you want my recommendation on keying software, I would say Blender. You could do this job in 5 min flat (if you knew what you where doing). Also, I hope you don't think I was ranting against you. Green screening is just a touchy topic for me. People think it is so easy when its not. So they try to do it with absolutely no research and their results look horrible, and a piece of blank white paper would legitimately look sooo much better. If you need more info, tips, whatever, I'll be around for the next few days.
