I had this exact same problem.
It took me ages to figure out but when I recreated my project from scratch in Vegas Pro 12, everything worked fine.
It would appear that at some point the project was trashed by Vegas Pro 11.
I could tell this as I had several VFX on my video events that I had removed (trials of product had expired and watermarks were present), but the watermarks were still visible.
Now, everything works great with only the occasional crash.
Darren