Menus are just mpg2 video, so you can create and render them just fine (in fact do a better job of it) in Vegas. That won't help the transitions though -- IMHO a lot of it is simple design tricks... look carefully on Hollywood movie DVDs. Often the best work uses the same philosophy as a magician -- distract the viewer so they're not paying attention. Sound rises in volume, then goes silent, sometimes almost violently, and you don't hear the pause. Transition video also has a bit of flash.

In short, design your menu video, then design your transition videos, and import them into DVDA. Use something like a mjpeg format avi if DVDA is going to render your menu & transition mpg2 -- If not, to keep DVDA from re-encoding, don't change the imported menu backgrounds, including deleting the title DVDA sticks there. You *might* get better performance putting your transition videos on menu pages as background video -- just add a button going where you want and have it auto-activate when the video's done.

As far as editing menus, the mpg2 video DVDA renders can be pulled out of the DVD and treated like any other mpg2 video. The button areas and highlights (sub-picture) tracks can be extracted, but working with them and re-use might not be worth your time and effort. Get software and guides at videohelp.com & doom9.org