I guess your formula is
=if(iserror(vlookup(a1,b5:f45,3,false)),0,vlookup(a1,b5:f45,3,false))
I don't think you can't simplify this formula or replace it by any other formula if you insist on having a 0 or blank when no value is associated with your lookup-value.
You can however just use =vlookup(a1,b5:f45,3,false) and use formulas that can cope with #N/A-values in further processing of the results.
May I also suggest that you can move your lookup table/range (d.i., b5:f45) to a separate sheet, then select and give a distinctive/meaningful name to your lookup table/range via the Name Box (or via the option Insert, Define, Name), e.g., CLIENTS, STUDENTS, DISCOUNTS, etc. Also, use if you wish 0 instead of false and ISNA instead of ISERROR. Then you can write the VLOOKUP-part as
VLOOKUP(a1,DISCOUNTS,3,0)
Aladin