Hey Mike,
Judging by your second post, you seem to have already figured it out.
The way I configured our system is to have both BP and Vendor number ranges external and I have the flag "Same" ticked for both directions. That worked for me.
I also added the BP number to the UI and the users can enter the free text value there. This copies it over to the Vendor Number in LFA1.
In fact, I have mimiced the Vendor Account Groups with Business Partner Groupings with the same name and what the users see on the scree are the fields Groupping and BP Number. They have corresponding Vendor Account Group and Vendor Number that are being created by the post-office processing.
We also have scenarios where the users would not select an account group with the initial request. The Data Specialist selects it during the enrichment step, then the approver finalizes the workflow.
We use several Account groups and they all have a corresponding BP Groupping with the same name and description, so the users won't make a difference.
Hope that helps...
Best regards,
Boris