I can't make the Data_value work

for I = 170 to 182
IF I = VLOOKUP(I, DayNr,1) THEN
something
ENDIF
Next

I = actual daynumber
DayNr is a range of Working day No.

If I substitute "I" with a value ( say 170), it works OK.
The goal is to decide, whether I is a working day or not.

How should it be written ?