Hi,
while doing Third Party @ MIRO, getting the annoying error "Purchase order number in document number already exists".
As per your statement are you trying to do Vendor billing in MIRO or you are creating a Sales order in VA01.
In the above what you wrote for VOV8 and OVAH is related to sales order save where it checks the unique combination of Sales doc type+Sold to+Cust PO number(VBKD-BSTKD).
Please confirm where you get the error also provide the error msg no.
Hope this is clear..
Thanks,
Suman$