Explain how is the vendor return processed without a purchase order reference?

First you have to observe the return column and then select — Item Detail > MIGO_GR > Goods Receipt for Purchase Order. Use movement type 161 if the intention is to deduct the stock otherwise, 162 is used to undo the changes. In the end, you have to ensure that the document is a return purchase order, and then the document is saved.
 You can also use transaction code M21N for this purpose