If the file is open then you don't need the path to it - you only need:

'[filename.xls]Sheet name'!range

In your VLOOKUP you are putting square brackets around everything, but
these only need to go around the filename, so it won't work. If R18
contains "052306" and the file 052306.xls is open, then the formula
should be:

=VLOOKUP(N16,INDIRECT("'["&R18&".xls]Sheet 1'!$A$2:$Z$26"),26,FALSE)

Hope this helps.

Pete