www.doom9.org has some great tutorials, and links to the best ripping tools. im quite new at the vob-->avi conversion, but im already quite used to it
what i recomend is this:
a program called 'dgmpgdec' that has a good 'quickstart' guide in it, where basically its an add on hack to 'virtual sub' to rip the video, and audio.... then to splice in the audio in *.mp3.wav format (to fool virtual sub) and keep the file size respectably low use 'wavemp3'
but first you have to rip the wave from the ac3 with 'lame' (a command line program that works great!)
also, there are bit rate calculators out there to make sure you dont have a rediulously small, or large file.
anyway, i think the tutorials will help more then i can.
good luck