Vlookup in conjunction with Index and CountA function

Hi all,

I am trying to create a table that will pull data off two spreadsheets with weekly sales data from 2014 and 2015. I am trying to set up the table so that it automatically pulls the most recent weekly sales data from the 2015 spreadsheet (did that already), and pulls the comparable data from the same period in 2014 (tricky part.) I was trying to use the vlookup function, searching for the last value input on the first row in the 2015 spreadsheet (which is just a "Week X" column), so that as new weeks are put in, it will search for the title "Week 21", on the 2014 sheet, and then pull the data in that specific store column, but as many times as I try its just telling me my formula is incomplete or something.

=VLOOKUP(INDEX('Weekly Sales (2015)'!A:A, COUNTA('Weekly Sales (2015)'!A:A), 1), 'Weekly Sales (2014)'!A1:AB54, 'Weekly Sales (2014)'!I:I)

That's the function I'm trying to use. Any suggestions, or an easier way to do this?

Thanks

Actually I just misunderstood what that field was supposed to be. I hoped it was something simple, I thought I was supposed to select the column like I did with the Index function, not list out the number of columns it was from the origin.

Thanks for your help!