Grazie,
I've been using PI3 for years as a stand alone app.
After selecting the desired particle emitter(s), I import my video file into PI as a background so that I can view the placement and motion of the emitter for the length of the shot.
I then remove the BG image file and render out the PI file as PNGs with alpha channel. This drops into Vegas (I'm still using 7.0d) as an image sequence on the track above the BG scene.
You can do the composite in PI, but I like the extra flexibility of separate elements...lets me change the visibility of the PI file if needed.
Hope this helps.
Ken
Same here, I use PI as a standalone app and render out as transparent alpha background .png image sequence.
Incorporating PI w/ an actual video is more involved.
The best way I've found to integrate w/ video is render the event that PI will be applied to from Vegas to .avi that PI can play back the video easily. It doesn't have to be great quality since you're just going to use the clip as a reference for the PI effect. Apply the PI effect over the video and export only the PI effect as .png and over lay it on the video in Vegas.
Grazie - I've also used PI as standalone for years and chose to -- not -- get it as a plugin for Vegas for it's far more limited in what one can do with it. As I recall I had some conversation about this at the CC PI board with Alan Lorence.