I have written most of my vlookup, but found that it is not robust enough to provide me the information I need. I am performing the following equation

=IF(ISERROR(VLOOKUP($A2,'[Backlog 10-5.xls]Old Backlog'!$A$2:$U$20000,9,FALSE))," ",(VLOOKUP($A2,'[Backlog 10-5.xls]Old Backlog'!$A$2:$U$20000,9,FALSE)))

What I have come to learn in developing this equation is that I can have 2 or more identical A2 column itemes, but to tell them apart, you need to use B2, which is a numerical value for the line number. How do I incorporate another Vlookup(or whatever function) that will say if A2 on worksheet one matches work Sheet 2, to then look at B2 and pull back/display on worksheet 1 the correct value from column number 9?