There is a simple reason for these crashes. I just went through the same thing and found that one of the files (a clip) that is part of the main project was not at the same frame rate as all the others. Hence, even though Sony claims to be able to handle that, you will run into issues at a certain project size. Also make sure your project properties are set to match the frame rate as well.
I discovered this the hard way. It seemed something had changed and all of a sudden I started getting those nasty aviplug.dll errors. I took the entire project off to another system that did not have Cineform on it. Same results. Hence, it purely had to do with frame rates. The change that happened and that looked as if some weird things came upon Vegas was simply that a file with a different frame rate was introduced to the project. Of course, if one does not pay attention to detail, then it may look like – “geez, all of a sudden the thing blew up for no reason.” – but as we all know, there is always a reason. 🙂