Double-check the Optimize DVD window to be sure that the project is set to AC3 and that the menu is as well. Also, where was the 5.1 audio created? 100% positive that it's 5.1?
I just tested in 2.0 and 2.0a: Created a simple menu, added a 5.1 AC3 rendered in Vegas, and preprared DVD files from this. Both times, the resulting menu still contained 5.1 audio (as reported by PowerDVD).