Or if you are not already committed to Adagio Purchase Orders, you could look at POTracker which allows you to create a purchase order for an RFQ Vendor or Blank Vendor, File the document and send it or email it to some vendors then when you select the vendor, you can retrieve the filed PO, change the vendor and create the actual purchase order.

Steve's or Patricia's suggestion should work too as I assume you could copy the PO and issue it to the successful vendor then delete the original.