Go to the first formula in your column, modify it with the ISNA argument, and
copy down. Hit F9 to recalculate if your workbook is not set to automatic
calculation.
You're missing some closing parentheses, around the ISNA() function and the
IF() function:
=If(isna(VLOOKUP(C3,'User Listing'!A,3,FALSE)),"",VLOOKUP(C3,'User
Listing'!A,3,FALSE))