VOB format is a subset of mpeg, so you should be able to combine the VOBs you want to read in just as you would mpeg data: by concatenating them together.
cat first.VOB second.VOB third.VOB | ffmpeg -i - outfile.mp4
Hoisting Matt Gallagher’s comment to increase longevity/visibility:
Newer versions of ffmpeg support concatenation as an operator on the input file. So you could use…
ffmpeg -i concat:VTS_01_0.VOB\|VTS_01_1.VOB\|VTS_01_2.VOB outfile.mp4