This works:

=VLOOKUP("TEST",'C:\Folder\[File.xls]Sheet1'!A:A,1,FALSE)

This does not:
(In cell C3) =CONCATENATE("'C:\Folder\[File.xls]Sheet1'!A:A")

=VLOOKUP("TEST",C3,1,FALSE)

=VLOOKUP("TEST",INDIRECT(C3),1,FALSE)

Is there a formula, function, or syntax I need to use to reference an
external file if the file is described in a cell?

TIA
Charlie