After a lot of attempts, I found a way to re-sync audio and video. Please excuse me if the explanation will be not easily understandable, with my bad English…
With MPEG Streamclip I choose to demux the original .vob files, and I obtained .m2v video and .m1a audio files (or .aiff files), as you suggested.
I imported the files in Toast, but in both ways (.m1a and .aiff) the audio was gradually out of sync.
Following the suggestion found in another forum, I converted the .vob files in .dv but burning those .dv files in Toast the DVD (or DVD image) was very bad, with the video full of distortions and vibrations.
But during the .dv conversion I see that some temp files appeared in the "Roxio converted items" folder in User->Documents: .m2v and .ac3 audio files.
I copied the .ac3 files in a new folder, together with the smaller .m2v files produced by MPEG streamclip.
I opened again Toast (Video window) and I add those files, then I saved a DVD image.
Now, after so many attempts, I have synchronized audio and video.
It seems that TOAST works bad with .m1v audio files (mpeg1)