Does kdenlive have a limit regarding the workable file-size?
Not sure about kdenlive, but when using Cinelerra the max file size was only limited by the file system (ie. ext3 upper limit). I would be surprised if you hit any limit, but that is depending on how you split up your clips. http://www.kdenlive.org/forum/working-many-gb-data-one-project has a bit more info, but I think your RAM is going to have more effect than any programmatic limitations.
If you are referring to the old windows limit of 2GB file sizes no this is not in effect on linux.
If you need to break up a large VOB file for prosessing, you can use ffmpeg (more details here: http://superuser.com/questions/31135/split-mpeg-video-from-command-line )