You may find this information useful, taken from wikipedia:-
"A VOB file (DVD-Video Object or Versioned Object Base) is a container format contained in DVD-Video media. It contains the actual Video, Audio, Subtitle (VobSubs) and Menu contents in stream form.
VOB files are encoded very much like standard MPEG-2 files. If the extension is changed from .vob to .mpg or .mpeg, the file is still readable and continues to hold all information, although most MPEG-2-capable players don't support subtitle tracks."
Original Article:-
http://en.wikipedia.org/wiki/Vob