The program vobsub2pgm does not exists in Debian 10 Buster.

With the vobsub2pgm program you can convert the subtitles from VobSub format (a subtitles.sub and an subtitles.idx file) to several imageNNNN.pgm images - one for each subtitle - and one image.srtx index file. Into the index file there is the start and the end timestamp for each subtitle.

vobsub2pgm -i $RIPDIR/VIDEO_TS/VTS_01_0.IFO subtitles image

The .IFO file is required to get the correct palette, width and hight, but it is not mandatory.