I think this may be the problem: DVDShrink is looking for VTS_01_1.VOB. But the titleset of the main movie goes something like VTS_02_x.VOB or VTS_03_x.VOB. DVDShrink is not very smart that way.
You didn't say what exactly you're trying to do. If there's no structural protection, there's no point in doing it like that. Now, you just could rename the files to make them work with DVDShrink. Or transfer the VOBs and the VTS_xx_x.IFO to a new folder and create the VIDEO_TS.IFO in DVDFab, the trouble is, DVDFab isn't smart either and you need a VTS_01_1.VOB or it goes stupid. That means renaming again.
So you see this doesn't really save any time. But the way to do it without confusion over renaming would be to rip JUST the VOBs (and if you like the VTS_xx_x.IFO), and create the IFOs in IFOEdit, which doesn't care whether or not there's a VTS_01_1.VOB. (You'd need to check each IFO afterwards that you have your preferred audio track selected to play first). Now, I can see doing this to bypass protection and get movie only, done it myself. But first rip movie files only, forget the other IFOs.
Good luck. And if I'm all wet, hey! whaddaya want?