Option B is the situation
You haven't provided any details of ranges, how your sheet is operated etc, so let's suppose your current formula is ...
=VLOOKUP(A1,$C$2:$D$10,2,0)
... then a few options might be:
1. The formula already given above, which would become:
=IF(ISNA(VLOOKUP(A1,$C$2:$D$10,2,0)),"instruction",VLOOKUP(A1,$C$2:$D$10,2,0))
2. =IF(ISNUMBER(MATCH(A1,$C$2:$C$10)),VLOOKUP(A1,$C$2:$D$10,2,0),"instruction")
3. Use Data Validation in A1 to stop the user entering an entry that is not in C2:C10 in the first place.