and I rendered it as a progressive-scan MPEG-2 with audio included in Vegas. Then I imported the video into DVD Architect and followed the standard template.
I don't know if DVDA's compressions bring in interlacing automatically or not. Or how to get rid of the interlacing if it does.
It sounds like he probably rendered a 24p MPEG-2 with 2-3 pulldown or whatever inserted. I've always assumed that means it contains those markers that instruct the player to double certain frames when displaying on a non-progressive player or television.
As far as DVDA goes, as long as it doesn't give you any notifications before burning, I'm pretty sure that means it isn't recompressing the material you've given it. Since you rendered using DVDA-compatible Vegas profiles, I don't see why it would use any compression of its own.
24 p is included in the DVD video standards, so you can render in 24 p in Vegas and compile it in 24 p in Architect without recompresson. Architect will put a 24P flag in the DVD. Any player will recognise it all the time.
I use it very often to get rid of interlacing artefacts.