vobsub is not a codec.
in fact what we call vobsub can alternatively be
1. Vobsub which is a utility that extract subtitle streams from the vobs found on a dvd. It produces two files (.sub and .idx). One of them eventually contains the subtitles as images (just like in the dvd), the other one has the timestamps to display the correct image at the correct time
2. DirectVobSub which is a subtitle display utility.
DirectVobSub is able to display subtitles created with vobsub as well as text-based subtitles.
What interests you here is DirectVobSub
both of them can be found on
http://vobsub.edensrising.com