Hi,
If you want to implement Self Billing for Third party, you have to do ERS first from MM perspective
in which you will do Manual MIGO and create a ERS billing with the External Reference number filled with Inbound delivery
once you create Self billing document at Vendor 1, you can refer the same same outbound delivery which will be saved as external number even in the Billing document
more information can be found uner
http://help.sap.com/saphelp_erp60_sp/helpdata/en/1b/ca82d840fe11d393930000e8214d78/frameset.htm
regards,
santosh