As mentioned, MPEG Streamclip is a free tool for demuxing VOBs. You will get a .m2v file and propably a .ac3 file.
If you got Final Cut Pro, you propably got Compressor. Compressor produces the new .ac3 files. You could then use DVD Studio Pro to mux 'em together (assuming you have that one, it's in the FCS package). Problem is now you have to re-author the DVD. DVDSP can read a VIDEO_TS though.
As for solution to just mux the VOB files, don't know, never had that problem. FFMpegX I guess does it. Toast maybe. Compressor has an option to mux MPEG2+AC3 into an MPG file, but I guess you would need another tool to get a VOB from here. VisualHub is a free one-click-don't-care tool, that produces VOBs, but I guess you cant just extract the file from the fileset and use it elsewhere - or can you?