I'm not very experienced in the 5.1 world, but recently did use Vegas 6 to create a 5.1 .ac3 file from a 5 mic recording I did earlier. I was then able to add it to a DVD with DVD Architect 3. This was a recording of a massive pipe organ, and the low bass thundered from the subwoofer. I used the default preset for DVD. Prior to that I tried customizing the preset to leave out the center channel, which I had not recorded. The 5 mics were 2 stereo pairs and 1 omin mic, which became the LFE source. By going away from the default setting for 5.1, my ac3's were resampled into crappy 2 channel sound that came out of the front speakers only, and took forever to resample.
By using the default preset within Vegas, DVD-Arch had the green check mark by the .ac3 file, and it wrote onto the DVD+R quickly. The sound was fabulous.
FWIW . .
Rick Z