There is no RMA function in QB, so anything you do is a work around

There is no connection between customers and vendors for things like this either

If you use the inventory item on the credit memo, it returns to stock
then if you use that same item on a vendor credit, it is removed from stock
when it comes back, enter a bill and apply the vendor credit - that returns it to stock

the hard part of the above is you really need to use the average cost of the item as the credit amount.  When it was returned to stock, that average cost was pulled out of COGS and assigned to the item, so you need it go back to COGS when you get the item to the customer, and that means a sale (see below)

If when you put it on the vendor credit you create a sales order for that customer with that item on it at a zero sales price, when you get it back create an invoice from the sales order (zero dollar) and get it to the customer

I add RMA to the sales order number so there is no confusion as to why it is zero dollar.