Well, thanks for helping me out but I'm about to give up: when I load the inverse telecined file in VirtualDub (the same file I used to make the preview, which looked FINE!) to make an AVI, it now shows scan lines again, for no !@$% reason!
So unless there is another completely different way of getting a lossless video file on Vegas that you're willing to share, I'm thiiiis close to giving up...