I have a project I am working on that has a number of Quicktime mov clips which use png compression so as to be able to use an alpha layer. This project will play OK in Vegas 9 (both 32bit and 64bit version) but will crash about 9 frames into a render. Rendering in Vegas 8 is fine. This looks to me like a bug.
I had a similar crash bug with Quicktime mov clips using the animation codec and an alpha channel. If I had manually changed their pixel aspect, they would cause a render lockup/crash when going to DV AVI in Vegas Pro 9, worked fine in 8.
SCS confirmed it's a bug. Yours may be similar. You should report it.