Is VLookup the best solution?

One of the reports I run produces a list of products, discounts and the number sold. However, the products and discounts are coded to 2 characters each. Some products can have any discount applied, some cannot. I can only get VLookup to work for products that can be discounted.

e.g:
AA:00 correctly returns 'Pie' (:00 = undiscounted)
AB:02 correctly returns 'Pie [Child]'
ZW:00 should return 'Soft Drink' but does not
ZW:01 should return 'Tap Water' but does not

I have tried to use ISNA in the formula, but it doesn't want to work! (I've attached a document to give you an idea of what I mean)
=IF(ISNA(B3),VLOOKUP(B3,$G$3:$H$10,2),VLOOKUP(LEFT(B3,2),$G$3:$H$10,2)&VLOOKUP(RIGHT(B3,3),$G$3:$H$10,2))

Your help would be much appreciated!

Thanks for the suggestions, but neither have worked. I'm using Excel 2003.