The "remove black background..." option has nothing to do with the alpha channel -- it affects the RGB channels only (and is the difference between a "straight" and "premultiplied" alpha).
If you select a codec that can support alpha (I like "Lagarith" or "Huffyuv"), check the "save alpha" option, and render, the output AVI will contain an alpha channel. You can verify this in pI3 by starting a new project, setting the background color to something non-black, then loading your rendered movie as the background image for a layer. You should see the background transparent.
I think this is probably some setting in Vegas that is set to "ignore" the alpha channel in the AVI. I don't use Vegas enough to know where those settings are without doing a lot of searching.
Alan.
www.wondertouch.com
