DVD architect i think will fill out a FULL DVD to max 4.3 Gb.....
and i dont think u should pre-render prior to insert into architect, because architect will then re-render again.
I am sure architect can accept DV format straight from your camera, add titles and menus and then get vegas to render to VOBs.
So if you want to add more to the DVD just add more clips in architect and it would reduce the bitrate to fill the DVD. (it increased your bitrate to go from 2Gb to 4 Gb)
Other guru will correct me on this if am incorrect