You can create filters in NPrinting using QlikView variables. It's in the ribbon in the same right next to Fields. See the following tutorial: Set a QlikView Variable to a Specific Value Using Filters
Or you can have a dynamic filter based on a QlikView expression. What is the logic behing vLastMonth? If it is something like month(now)-1, then you can use that syntax in the value column, and not use the variable at all. You would have to check "Evaluate" if using this method.