I had a lot of strange Vegas behaviors with QT media on the timeline, broken frames on QT export, and the QT Render dialog freezing. Uninstalling latest Quicktime and loading 7.4.5 appears to have solved all problems. And remember to uninstall/disable the Apple Software Updater after installing.

Available at..
Download QuickTime 7.4.5.67 (http://www.oldversion.com/download-QuickTime-7.4.5.67.html)

/magnus