You can use VOB Edit to join split VOB’s, it is totally free. You can download it at doom9.net
Or u can download ChopperXP, also totally free, to split VOB’s, or just cut out sections u want from it without loosing any quality. (IM not sure if u can join VOB’s, but try it and see, becuase this program is MUCH easier to use than VOB Edit.
Both of these programs keep the VOB intact, meaning u will still have the Subtitles, and all the launguages it has. Even when cutting, the output VOB will have all of these, hope this helps.