You can open a second instance of Vegas, then copy/paste between the first instance and the second. I can't remember if the markers and regions copy/paste as well, but there are scripts that will export/import markers and regions.
In Vegas 6, you can save the first project as a VEG file and then import that file into your second project. Same comment as above for the markers and regions.
Yo - Cools! - Look, I don't KNOW what you are trying to finally achieve - but . . If you are just wanting to swap event-content, save your existing worked on project and then save it as the NewProj.veg and then add "as takes" to this project's events positions. This way you can audition the alternative takes in an already region and marker infested project - any good?