never done this
might be possible to join the vob files together with vobedit and ifoedit and then transcode down from that.
12: Rename the VOB's you want to merge together as VTS_01_001, VTS_01_002, VTS_01_003, VTS_01_004 ect.
13 Open the first VOB in VOBedit
14: Click "Edit-Join Clips"
15: Next you will be prompted for the destination directory for the combined files. The program will start combining the files
16: Once combined, you can close VOBedit
17: Run IFOedit and select "Create IFO's"
18: Select your VIDEO_TS directory and your 1st VOB file for your new IFO files for the "Output Stream" box. Keep the destination directory the "Same as source"
19: Once done, close IFOedit and open IFOedit again.
20: Open your VIDEO_TS.IFO file and select "Get VTS Sectors" click OK until done.
21: You may now close IFOedit .
the above was taken from a tutorial from dvdrhelp