Can the background of a video track be made transparent or the color changed?

You can try dropping the ChromaKey FX on it and selecting the background colour instead of the usual green or blue.

Thanks Mike, actually I should first ask if there is a video format that can have transparent areas in it. I'm using Digital Juice backgrounds and the samples in many cases do not have black areas, but other solid colors, and I can't figure out how to achieve that.

I found the answer! Vegas does not automatically detect the correct properties for digital juice overlays, got a simple fix. They are now transparent and beautiful.

As I recall, Digital Juice gives you an alpha channel option when you render some of their files.
I know I've rendered QuickTimes and PNG sequences (maybe AVI but I can't remember) with alpha and they work.
I'm not at my Vegas computer right now but, as I recall, you may need to select Properties and then one of the 4 options (Premultiplied, dirty, etc.) for it to work properly.

edit: you beat me to it!! Glad you figured it out :-)

thanks Mike, I will look for the rendering option also! (I did check that option, it's already checked...no help)