Vegas does not have any built-in facility for calculating bit rates, tough if you have your project already rendered to avi, you can use DVDAs fit to disc function to give you a good idea.
DVDA has never been particularly good at knowing how many MBs the media adds up to, though DVDA4 is better at this. For a dual layer, I’d recommend keeping it under 8 GB, and that includes your audio files too.
Even if DVDA reads a higher GB total, you can still try and prepare/burn. It may squeak by.
Gary Kleiner
Learn Vegas and DVD Architect
http://www.VegasTrainingAndTools.com