In the scenario of 4 orders:

1st: Original SO (to external customer).

2nd: Direct delivery PO

3rd: Intercompany SO

4th: Related Direct delivery PO (from the IC SO)

When the external role for the user (Vendor admin external and Vendor contact external) and while accepting the direct delivery PO through the vendor collaboration portal, error message: "The intercompany vendor is not found for customer xxx"

A not friendly solution: is to let an Admin user log in to system and accept the PO or the delivery type changed to Stock instead of Direct delivery.