Have you tried viewing the sequence in another application? That will tell us if the problem is with the output file itself, or with a Vegas setting or something else.
If you don’t have another application, just load the sequence into particleIllusion as a background image for a layer and turn on transparency. If the alpha is wrong there, try a new particleIllusion project with a much smaller stage size, like 320 x 240 and save with the same alpha settings as before. If this small project works fine, then the problem is most likely with pIllusionRender, the app that launches for large renders.
First thing is to make sure you’re using the latest version (check the downloads – updates page on the wondertouch website). If you are, then launch pIllusionRender manually, and click the “options” button and turn off hardware acceleration.
If the problem is not in pIRender (the images look fine when viewed with alpha in another app or particleIllusion) then it’s just a matter of getting the setting correct in Vegas.
Alan.
http://www.wondertouch.com