wants another left parenthesis at the end?
Yes, you're missing a closing parenthesis for the ISNA function. Also, what
type of lookup are you wanting to do? Do you want to find *exact* matches
only? Or, do you want to find "approximate" matches?
For exact matches only:
=IF(ISNA(VLOOKUP(A42,County_by_City,3,0)),0,VLOOKUP(A42,County_by_City,3,0))
For "approximate" matches:
=IF(ISNA(VLOOKUP(A42,County_by_City,3)),0,VLOOKUP(A42,County_by_City,3))