It’s a matter of personal preference whether you render to AVI or MPEG.
I prefer to render to MPEG from Vegas as this gives me a lot more control over the bitrate, quality level, etc.
You said I render to DVDA 4.0 format to save time.
What template are you using?
I always start with the DVD Architect NTSC video stream template for video and and then modify it as desired.
Audio is the AC-3 Stereo DVD template.