Try this (untested):

=IF(VLOOKUP(A5,'Z:\Information Analysis\CSP Study Log Development\CSP Reporting\Excel Solution\[CSPProject.xls]Sheet1'!A:N,13,FALSE),"YES",IF(VLOOKUP(A5,'Z:\Info rmation Analysis\CSP Study Log Development\CSP Reporting\Excel Solution\[CSPProject.xls]Sheet1'!A:N,13,FALSE)="","Not Assigned","NO"))




It's quite a long formula so I'd be tempted to use a helper column instead.
In the helper column you could simply look up the value:
=VLOOKUP(A5,'Z:\Information Analysis\CSP Study Log Development\CSP Reporting\Excel Solution\[CSPProject.xls]Sheet1'!A:N,13,FALSE)

Then you can reference the helper column with the IF() conditions. Say the helper column is column B:
=IF(B5,"YES",IF(B5="","Not Assigned","NO"))



Also, I notice you are returning the value from the 13th column. If that's the case the lookup table can be reduced to A:M rather than A:N.

Hope that helps...