I recently changed computers as well as Sony Vegas iterations (I went from Pro 9 to Pro 10), and as I was opening up the first Vegas 9 file (opening it with Vegas 10), it told me that all the media couldn't be found, etc., which I expected. I searched for all the videos and Vegas found them with no problem (on the external hard drive on which I had backed up all my files).
However, some files are listed as "Offline Media Files" in the Project Media window, and I can't view them in the timeline when I try to play them. Upon further investigation, I realized that all the files in question that are listed as offline are either .avi or .mov files. Does Vegas Pro 10 not support those file types? It seems weird that Vegas Pro 9 does and Vegas Pro 10 does not.