Unfortunately, you were not specific about the Quicktime files you are adding. Quicktime files, like .AVI files can have many different compression types.
I am going to take a guess and say that you might be trying to edit files from a consumer MPEG4 camcorder. It could be the Go-HD, or the Kodak Zi6 or any other number of the ones that use MPEG 4 AVC compression and AAC (LC) audio. They will crash Vegas every time.