Don't know version you're asking about, but I can comment on DVDA-2:
Encoding options are limited in DVDA-2. You can specify a bitrate under "Optimize" when preparing the DVD, but I don't believe that there is an option to use VBR.
If you're a Vegas user, you'll find much more adjustability of encoding options at your disposal. Many users prefer to do all their rendering from Vegas for this reason.