[Xavier Dolz] "Why is this happening?"
Because Vegas doesn't support editing Xvid, DivX, or any x.264 in an AVI container. This is a delivery format that is not good for editing.
[Xavier Dolz] Any idea about how this can be solved?"
Transcode these files into a better editing codec like MJPEG. You could also uninstall ffmpeg since this may be causing Vegas to not interpret the files correctly. I have used x264vfw without ffmpeg and not had any problems.
~jr
http://www.johnrofrano.com
http://www.vasst.com 
