In VMS, you have specific shapes to use via "Cookie Cutter". Vegas Pro has alternative choices.

1) Video A in track 1
2) Add Cookie Cutter to Video A (e.g. a circle)
3) Video B on track 2.

This will yield an inner circle with Track 1 over Track 2.

Ah, so we can't chose our own shape only the preset ones?

But thanks a lot Markk655, that helped :)

You can, at least starting with version 6 (i think).

Add a graphic of your desired shape to the top track, then add the two videos on the next two lower tracks. Join the upper video track to the graphic track as a compositing child and set the type to "multiply/mask". Add the mask generator effect to the graphic track.

Now, wherever your graphic is white, you'll see the upper video, and wherever it's black you'll see the lower video. Your shape can be anything arbitrary that you draw. You can even use a video file for an animated mask.

I did a little tutorial here: http://www.rchv.com/store/detail?tp01 ... it's for Vegas Pro 3, but it should work the same in Vegas Studio. Note that the mask generator effect wasn't required back then so it's not in the tutorial, but you will have to add it.

Some other good links -

here , here

You'll see the same tip discussed if you use the keyword "mask" or "compositing" in the search function.