Within the attached array formula, I need to either perform a lookup, or use
the data I have per iteration, to count for various values in column B.
However, as I understand, VLOOKUP requires a single value or reference as the
lookup_value. I have tried using the INDEX function (see below), but it
appears to only pick up row 1 for each iteration, hence not matching with any
valid lookup values.

How do I, inside an array calculation, specify the row which has been found
to match the preceding IF statements?

Effectively, I wish to perform a lookup on another workbook, using column C
and the row which has been found to match the two preceding IF statements.

=SUM(IF($A$1:$A$2000="OMS",IF($B$1:$B$2000="",IF(V LOOKUP(INDEX($A$1:$C$2000,ROW($1:$2000),3),'[Lookupfile.xls]Name'!$A$1:$B$2000,2,FALSE)=$B9,1,0),IF($B9=$B$1:$ B$2000,1,0)),0))

regards,
Pete.