you will have to get the ac3 on your computer. you can do this either before or after you rip the DVD. To get the ac3 from the VIDEO_TS folder, use PGCdemux as Moth3r suggested.

after that its besweet time. you choose ac3 to six wavs. and hit ac3 to wav (after setting the input and output). this will give you 6 wav files that you can import into Vegas. You will have to assign them to the appropriate channels there.

Correct, you really dont need audio on yor video file since you have the 6 wav files.

For output, I would suggest frameserving to a high quality encoder like CCE or TMPGEnc. All of the frameserving directions are in the link I provided above.

I would suggest exporting the video as 1 file. NOT scene by scene.