Well, I'd de-mux the VOBs out into their component MPEG-2 video and PCM/AC3 audio streams, and then re-build the DVD in DVD Studio Pro. Can iDVD import already-encoded MPEG-2 without re-encoding it? I've never used it.
Toast can take DVD-spec MPEG-2 without re-encoding it and let you set up a menu, I think... But the first step is to de-mux those streams. I'd recommend using MPEG Streamclip.