The Vendor Returns button is located in the "Inventory Center" of the Retail tab.
Vendor returns occur quite often and allow a store to recoup money invested in buying new stock. If a store has too much of a certain item in stock, often they can send it back to a manufacturer for a refund.
After clicking on the Vendor Returns button you'll be brought to a page that looks like the one below. The top portion displays all orders for the currently selected store. Depending on your permissions level you will most likely only be able to see vendor returns for your location and not be able to select any others. Each vendor return shows the percentage complete (based on the return's total amount compared to the amount that has been processed), the return totals and quantities, the PO number to use when sending items out of inventory, and the vendor name. The bottom portion shows the items included in the selected vendor return. When beginning a vendor return you must click the "Activate" button in the lower-right corner of the page before you can start scanning items out of inventory.
Once a return is activated you can print out a list similar to a pick list. Aside from the usual information (quantities, titles/artists, barcodes) there is also a barcode with the PO number used to send the items out of inventory.