Using Odoo Studio:
1. Create a new 'Many2many' field (related to 'product.product') on the Purchase Order model.
* Name it like vendor_products.
* Set it as computed, with a dependency on 'partner_id'.
* In the compute method, search for products where the selected vendor ('partner_id') is listed in the product's vendor list ('seller_ids').
* Mark this field as invisible on the form.
2. Add a domain to the product field in the Purchase Order Line model.
[('id', 'in', parent.vendor_products)]
Let me know if you'd like a demo feel free to contact me via WhatsApp at 7538824350 or send me a DM.
With this change you can no longer order generic products such as office supplies, janitorial supplies, warehouse packaging supplies, consumable computer supplies, etc.
Most of the time, this change is requested because there is a perception people will order the wrong things from the wrong Vendor but in reality it rarely happens.
The Vendor will tell you straight away "we don't sell that".