Thanks Nick. I should have said that I was referring to HD video and I have never used any of the Pro versions of Vegas or DVDA, so at present, for me the answer seems to be no.
Yes it's strange that AC3 audio exported with either of the Vegas MS14 (or 13) templates isn't accepted by DVD Architect but needs to be recompressed. Even stranger is that you cannot drag the AC3 files to MS timeline or even add them from Explorer to your Project Media. It just doesn't recognise them as valid media yet it created them!