With Vendor Managed Inventory (VMI), inventory management and replenishment planning and thus the responsibility for the supply of certain articles is transferred to the supplier.
This requires the regular transmission of stock or consumption data to the supplier.
For this the Vendor Collaboration Portal provides the necessary functionalities as the Vendor can monitor the available inventory via form On-Hand consignment inventory.
Furthermore, it provides the option for the Vendor to overwatch the consumption via form Purchase orders consuming consignment inventory and Products received from consignment inventory and to issue Invoices based on the agreements between the two companies.

What’s missing in this context is, that the Vendor can actively create replenishment orders. The Vendor should be able to push goods to the stock as he is per concept responsible for Inventory management and replenishment planning.

So, it would be necessary to add a functionality in the Vendor Collaboration Portal/Consignment inventory module which enables the Vendor to create replenishment orders.
Same as for the concept with the Invoices where a new Pending vendor Invoice is created, this should generate a new Replenishment order in Procurement and sourcing/Consignment.
As soon as the inventory arrives physically the buying company can post the product receipt.

Adding this functionality to the Vendor Collaboration Portal would complete the business process.