Hello to the Community, first time poster in this particular Group.
I am a long time Magix user (Samplitude) and Vegas user as well, going back to version 4 (Sony).
The NORMAL AVI converter plugin (found in File IO folder/avi) for AVI files in Vegas is not very good, aviplug.dll. A replacement is found here,
http://originaldll.com/file/aviplug.dll/38545.html
and, of course, YMMV, (and moderators, this is not spam or virus, please check it out yourselves) but this has opened so many files for me, when Vegas will just hang otherwise. Download and replace the current file - aviplug.dll - with the new one (it uses the same name, so I always just rename old one with x in front - so you can always revert back - the old one will be about 600 odd bytes, and the new one 1152 bytes).
NOTE, the when you update/upgrade VEGAS PRO, you will have to RENAME the older file again, (and hence the reason for my post, save you some grief) as Vegas update package will implant a new "old one" and your 1152 byte file will still be there but now just a (2) redundant version and not the default, so your files won't load as they did before until you RENAME again the 600 byte file.
Hope this saves someone some headaches!! It sure helped mine...Cheers.