Combining projects is very simpel: Just create your sub projects in independent .veg files. At the end you can create a master file where you drag al the sub .veg files onto the timeline. You can look in the manual for NESTING
The advantage of this method over the possibility to just render the sub -parts of the project to a new track (Ctrl M) is that you always can adjust a sub-project and the master project file will be adjusted automaticly.
‘I have discovered with Vegas 9c that the longer the project (30+minutes) the more it crashes’
What kind of files are you working with? Are you working with Vegas 32bit or 64 bit?
Theo