You are right it's not possible directly because a menu is strongly tied to the movie that is in the vob and you can just reencode the vob and take the old menu :
1/ if you take the chapter selection menu, the VTS vector must be realigned (the chapter points)
2/ if you take the audio selection menu you must have the same audio streams in the same order than on your original
3/ if you take ony the main menu, you have to edit the information with ifoedit to have them represent exactly the movie. But this is possible and should be tested.
But as we are talking about that, let me introduce you DVD Remake, the successor or MenuEdit. This tool allows to edit a pre-existant menu (ripped from a DVD) and de-activate all buttons that are useless (bonus for instance).
Very usefull with a DVDShrink Ripp.
http://www.dimadsoft.com/products.php
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