Yes, you must use Main Concept but if you create an AC3 movie you should not encode audio stream on Main Concept ( create separate with Vegas Video AC3 )
Well, this is really stupid then. Why would they limit it only to MainConcept ? Seems to me that SF will be losing a lot of potential buyers because of this.
I haven't been able to test this, but I don't think you have to use the MC codec. I think the important thing is that you need a program (muxed) stream, not an elementary stream. See page 15 of the manual.
In order for DVDA to recognise the stream just use TMPGenc tools to multiplex the CCE video stream with a dummy audio steam (ie. give it no audio stream). This will create a MPG file with only the CCE video. DVDA will now load the MPG file, you can then change the audio to a seperate AC3 file.