Neat concept, bad example.
There’s nothing more wrong than a website that starts blasting music at full volume as soon as it loads.
But, you asked about formats…
Consider that the SlideshowPro website says that “The SlideShowPro Player for Flash component requires one of the following versions of the Flash authoring application (Macintosh or Windows): Adobe Flash CS3 (and up), Macromedia Flash 8, or Macromedia Flash MX 2004.”
It would seem to me that you would create the best quality file you can from Vegas and let Adobe Flash encode the appropriate FLV file for you.
Steve Mann
MannMade Digital Video
http://www.mmdv.com