I’ve been struggling with Sony Vegas for about two years when it stopped opening avi encoded with different codes, and also mp3, mov and most of mp4.
Even though I got K-Lite codec pack installed in the system, and all other programs have no trouble opening said formats. First I thought this is because I use a portable conversion of Vegas not installed into the system. However, turns out this problem exist even on properly installed Sony Vegas. Then I thought this is because Vegas is x64 bit, but K-Lite seems to incorporate both x32 and x64 bit versions of the same codes. You drag the file to the time line – and the cursor turns into a crossed circle and that’s it. So I uninstalled K-Lite and installed Xvid. And finally – both mp4, mp3 and avi containers now can be opened by Sony Vegas as they should! And for mov, it at least shows that QuickTime is not installed, even though K-Lite has QuickTime Alternative build in. So fuck K-Lite, go Xvid. Or ffdshow. You can also install now years old QT Lite QuickTime Alternative, it works perfectly fine in my Windows 8.1 64bit with Sony Vegas x64.