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 VOB containing a feature video).