If you didn't get DVD Architect along with Vegas, you really should consider it. Even my 3.0 version of DVD Architect Studio has "professional level" controls for virtually every aspect of authoring. In addition to the ability to create custom menus and multimedia playback, I can use my best quality AVI's for video sources and control the bit rates for compression either globally or for each source independentally. This allows me to put the main program at a higher bit rate than the DVD extras. And I can also create a DVD Plus disc for use in both computers & DVD players with ease. I can't imagine DVD authoring without these controls.
Since my source video is in my least compressed video format, I don't have to calculate the bit rate at all. The software tells me when I'm packing in too much video for my selected bit rate, then can automatically optimize it to fit the disc. Or as I mentioned, I can change bit rates on files individually until it all fits the disc. My biggest complaint is that I have to upgrade to the Pro version to be able to add in alternate audio and/or video. (It's just one more reason to go Pro.)
But anyway, to answer your latest question about a bit rate calculator, here's a link to part of a personally recommended series of articles, http://www.createspace.com/bitbudget It is provided as part of a series of articles on "DVD Authoring Nightmares" they will email to you once you register at http://www.createspace.com/Support/HowTo/Authoring.jsp
Have fun with your authoring & good luck!