Finding a match in different columns

Hi - I'd appreciate some help with a formula to do the following in Excel 2003...

If a number in cell M1 is also anywhere in column A (column A has unique numbers), then cell W1 (date) should equal the content of cell V (also date) in the row that that number appears in column A. If M1 is blank (or there is no match in column A), then W1 should equal V1.

In W1, I currently have...

=IF(VLOOKUP(M1,A:A,1,FALSE),V1,VLOOKUP(M1,A:A,1,FALSE))

but that's returning the value of V1 in W1

Any help would be much appreciated, thanks