DVDShrink needs a proper DVD-Video structure which a single VOB file does not make. There needs to be the associated IFO and BUP files too.
Your best bet would be to rename the file to .mpg and get IFOEdit
to create a proper DVD-Video structure for the file and then Nero will accept it. Alternatively, get the demo version of TMPGEnc DVD Author (which will also burn the disk).
The reason Nero is complaining is because you are trying to compile a DVD with a file structure it does not understand.