Creating video mask

[MI6]

I want to create mask from video (AVI) file but I don’t know how.
I have captured video file from Windows Media Player (visualizations)
with colourful motions and I like to create layer mask but with transparent
center (about 70% transparent elipse in center) so the main video file (parent file)to be visible in that elipse. My main question is how to produce such a video file with
transparent elipse. It is possible with Vegas or Premiere. I have tried to read the help file in Vegas about generating mask but there is no explanation for my idea.
Regards, MI6.

[Richie V]

It can easily be done in Vegas using the cookie cutter function. Apply the cookie cutter to the visualisations track (track 1) and tweek the settings to how you want, then have the 'main video file' on a track (track 2) underneath.