What codec are you rendering your AVI in? I suggest DivX or xvid, Huffyuv, or Lagarith. Make sure you’re using an 8-bit codec too, not something that supports 16bpp or higher, which is unsupported in particleIllusion.
I’ve never seen the “movie bitmap not populated” error, so I expect it’s related to the codec you’re using.
You can always render as JPG image sequence and use that in particleIllusion.
You’ll probably use the footage in particleIllusion as reference only, then remove it and do your final render of particles alone with alpha, for final compositing in Vegas.
Alan.
http://www.wondertouch.com