Hello,
I hope you and your family are doing well.
In the past I specified the primary vendor on the item record as a default vendor. Additional secondary vendors for dual sourcing, I referenced as approved vendor via approved vendor list on the released product form and the Purchase tab on the Actionpane as well as added the external item description/number per item/vendor relationship on the released product form and the Purchase tab on the Actionpane.
If you create PO's manually, you specify the correct vendor on the PO header at time of creation. If you are leveraging PRs or master planning and planned order to create the PO the primary vendor will default in from the item record. You can edit the vendor record e.g. on the planned order form prior firming. You could also adjust the vendor account on the PO header after firming the planned order if required.
Thank you!
Best Wishes,
Chris Segurado