Not familiar with Sony Vegas or Architect, but, you say you are burning to DVD. Are you? or does the programme think you are, in which case your rendered video is probably a bit bigger than the limit of 4.7 GB.
Is there a default folder that Architect uses? If so, where is it?
Are you saving as HD or AVI - there is a difference in file size.
How big is your file?
Also, do you have SP3 installed?