Such question should be raised based on the the specific business case with the responsible people from your tax department.
The SAP note explains how the SAP design is, from where the defaults for the purchase order is coming and where it can be overwritten, it even offers an option with a BADI to manipulate the determination.
Tax is not that straight forward like we all wish it to be. In general I agree it should come from the plant, but you actually raised the requirement that it shall be from the vendor. So there might be a certain business case that may require an exception from the rule, maybe because you pickup the goods instead of getting it delivered.