[Paul Gregory] "I noticed that one had an audio track names as Microsoft Sound Mapper. I'm not sure just what that is but this isn't listed against the other veg files when they are open. Could this create problems?"
No sure. Try importing all of the other projects and see if leaving this out helps. I know Vegas has to build an audio proxy for each of the projects so if there is an audio problem in one of them it can cause problems.
[Paul Gregory] "It's making me ask what if any other advantages are there with using the nested veg files?"
I use them for intros and lower thirds and isolating 3D Track Motion and they come in quite handy for keeping the main timeline neat and clean. I also build my training DVD's with each chapter being a nested project and I've not had any problems. Perhaps you are adding more nested projects then your computer can handle?
~jr
http://www.johnrofrano.com
http://www.vasst.com 
