Lets say you have menu A and you have a menu item that takes the viewer to menu B: In order to have that transition be a 'page turn' for instance:P
You make a still copy of menu A and a still copy of menu B. You take both into Vegas and make a short movie with A doing a page turn into B.
Then back in DVDA you have the menu B item on the first menu (A) link to this new movie. So you click "Menu B" amd you see menu A page turn to menu B. You then set the end action of this short movie to link to menu B. It will not be totally smooth but I believe that it is the only way to get it done in DVDA. If someone knows a better way, I'm all ears.