if u still have the original mpg file,you could try a program like
subrip to ripp the subtitles you want from the vob's and an app like WinSubMux.exe to add the subs to the mpg file.
search in doom9.org for the software.
http://www.doom9.org/software2.htm#subs
if still not sure how to do it,,,download and install, again from doom9
dvd2svcd and load up your dvd movie or ripped vob's from the harddrive as if you are going to ripp the movie again,,and stop the process once the subtitles are ripped. this will put the subtitles as bmp's and give you the txt files needed to import them into winsubmux.
ztr