Excel 2007 : Calculating days with key

[pansovic]

Each time I create an invoice I want to make a follow up on the payment of that invoice. So I want to calculate in how many days from due date the client has paid.
So, I want that when I type in the invoice number it automatically takes the due date of that invoice and makes the calculation of days. I was thinking with a VLOOKUP formula but cannot figure it out.
See attached the document.