There is a free codec information tool available which is called "MediaInfo". Any chance to load this certain file "ABC.avi" into MediaInfo and report what infos MediaInfos lists for "ABC.avi"?
Launching the MedioInfo EXE file and drag & drop that file "ABC.avi" onto the title bar of the MediaInfo window. Or from the MediaInfo menu choose "File/Open/File" to navigate to "ABC.avi".
You use a file format which is not natively supported by Vegas Pro (which is h.264 video and MP3 audio in an AVI container).
So you would either need to transcode these files to a different format before you try to import into Vegas Pro. Or install an appropriate codec in Windows (but I don't know which codec this could be in your case, maybe x264vfw). Sorry, for this particular file format I can't help too much because I never use h.264 wrapped in AVI.