COMBINED 'IF' AND 'VLOOKUP' FORMULA

hi 

I'm trying to create a VLOOKUP which compares 2 excel spreadsheets and provides the value that does not appear in one for the spreadsheets.

eg Comparing spreadsheet1 against spreadsheet2 to see if 'SS1A' is present in SS2 and if it isn't, it places the value of SS1A in a separate spreadsheet to show that it is still outstanding.

I realise that i need to create a combined IF and VLOOKUP formula but I am struggling.

Currently i am managing the formula to provide me with data that is in both spreadsheets when i need the opposite

Any help would be appreciated.

Thanks

Tamy