|Once again without seeing your actual workbook, I am limited in what suggestions I can offer.|
First, if you are using the exact formula that you have posted and are dragging it down, let say in Column D, then your lookup_array is changing in each cell.
You need to use Absolute Referencing to keep your lookup_array constant. Highlight B1:C20000 in your first formula and hit F4. You should see this:
Now when you drag it down, the $B$1:$C$20000 will not change.
Second, since you have omitted the range_lookup argument, it defaults to TRUE meaning that it will try to find an approximate match if it can't find an exact match. Read the VLOOKUP Help file in Excel to see how it handles what it calls an "approximate match".
If you want it to find only exact matches, use this:
Finally, if you don't want to see the #N/A error should one legally occur, you can use something like this:
"No Match Found",VLOOKUP(A1,$B$1:$C$20000,2,0))
I hope something here helps.