I am experiencing same problem. whenever I try to open MP4, Vegas crash. I am using windows XP 32 bit. Vegas 8.0c I have quick time 7 pro installed in my PC
I was using Itunes 9.1. I downgraded to 8. Still it didn't worked. I couldn't get AVIDemux to work. So what I did was, downloaded the trial version of Corel VideoStudio Pro X3 and converted the video into AVI. Then opened in Vegas. Looks like Vegas is not fully compactable with mp4 files