You cannot USE the bag before you get on the plane home if you want to get the VAT back. You have to show the sales associate your passport in the store, and they will give you a form to get stamped at the airport. The VAT official will only stamp your paper if the items are unopened and unused and clearly still new. If you show up using your new purse, they will not stamp the form. Then you can either put it in the box for slower processing, or bring it to the cash lane, and they will take a percentage of your refund, but give you cash right away.
There is a museum on the top floor of LV on the Champs Elysees. There is a door to the north of the main entrance.