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Video Title Set Menu (VTSM)
The VTSM is a regular VOB that displays the menus. If the DVD title has a menu (I'm guessing here that it's optional), then the filename is the first VOB.
So for the first title, it would be VTS_01_0.VOB. You can generally confirm this by looking at the size of the first IFOS, since they are generally very small in size (compared to a feature).
