MEncoder is capable of extracting subtitles from a DVD into VOBsub formatted files. They consist of a pair of files ending in .idx and .sub and are usually packaged in a single .rar archive. MPlayer can play these with the -vobsub and -vobsubid options.
You specify the basename (i.e without the .idx or .sub extension) of the output files with -vobsubout and the index for this subtitle in the resulting files with -vobsuboutindex.
If the input is not from a DVD you should use -ifo to indicate the .ifo file needed to construct the resulting .idx file.
If the input is not from a DVD and you do not have the .ifo file you will need to use the -vobsubid option to let it know what language id to put in the .idx file.
Each run will append the running subtitle if the .idx and .sub files already exist. So you should remove any before starting.
Example 12.3. Copying two subtitles from a DVD while doing two pass encoding
rm subtitles.idx subtitles.sub mencoder dvd://1 -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=1 -vobsubout subtitles -vobsuboutindex 0 -sid 2 mencoder dvd://1 -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=2 -vobsubout subtitles -vobsuboutindex 1 -sid 5