AC3 files rendered from Vegas 14 Pro are not compatible with DVD Architect 7 Blu-ray. They are always recompressed in DVDA7. This does not matter for Stereo AC3 (although it's a waste of time) but 5.1 Surround AC3 files are recompressed to stereo. Blue-ray players indicate that the audio is 5.1 but it is not - there are no Center, LFE or Rear Channels. In other words, it is impossible to make Blu-ray surround AC3 audio tracks in DVD Architect 7.
This does NOT apply to DVDs. DVD Architect 7 accepts the exact same files and does not recompress and reproduces 5.1 surround properly.
I put in a report to Magix (Ticket 2017053117000371) but they sent back a boilerplate "How to.." e-mail which did not address the issue and then closed the Ticket.