I'm not even sure if VLOOKUP is what I should use...

The spreadsheet I am working with has multiple columns containing accounts
payable information: invoice numbers, costs, invoice dates, and a code for
the buyer that purchased the items on the invoice.

The information is coming from another program which someone kindly
transfers to excel for me. The problem is that each buyer has more than one
"buyer code"...for example John Smith is buyer code AA, B2, CC, and D1. So -
right now - all I can sort by is this buyer code - which isn't very helpful
if I am looking for total invoices for a buyer. Some buyers have three codes,
some just one. There are about twenty buyers, and about thirty or fourty
codes. I would like to use the buyer code to assign the buyer name in the
same row. I don't think VLOOKUP is what I need...and plus I can't get it to
work.

Any suggestions? There are too many buyers/code for nesting IF functions or
even a concatenate function. I think, also the use of the type of buyer code
(AA, B1, C2) confuses the function.