To get the main feature in a single .vob file, select the "Mode" tab and select "Title Only Extraction" from the pull-down menu. MacTheRipper will select the longest feature as the main feature ("MP"), although you can select other features. You can rename the resulting .vob file as an .mpeg file because it is an MPEG-2 file. (Does anyone know if there are there exceptions, where the .vob file is not MPEG-2?) I use VisualHub to convert to other formats as needed.
Edit to add: I usually open the .vob file in MPEG Streamclip to check for timecode breaks (Edit > Fix Timecode Breaks, then File > Convert to MPEG... to save the fixed file).