Automatic vendor selection in PR

Hi,

we have one Scenario:

Our Client has 2 kind of customer divided on zone basis North and south. Requirement is like :

IF the Sales order comes comes from North zone customers : group company (A) will deliver the stock to the customer

and if Sales order comes from from South zone customer : External vendor (B) will deliver to the customer

After Sales order creation the PR will automatically generate by the system and based on the Customer zone (North and south) the system will automatically determine the vendor (A or B).

Please suggest how to Proceed with this scenario in SAP?

Thanks & Regards