try this.
divide the project in two and make 2 Regions then save. for example if you have 2 minutes video on the timeline, make region for 0-1 minutes and the second region 1-2 minutes. Render the first region. open another instance of Vegas pro and the same project. render the second region.
check the task manager if cpu increase to 40%.
when the render is complete, create another video track on top, put the 2 rendered files and render it again to make it continuous 4 minutes rendered video clip. no worry for render generation because of smart render for avchd.
I have AMD Phenom II X4 system with 8 Gig of RAM. Vegas pro 10e
if i notice the cpu usage is very low, i chunk the project in to 2-4 render.
hope this helps
thanks