Not all platforms have variants associated with a supplier (called a vendor in Inventory Planner). If that information cannot be synced to Inventory Planner then you will need to set up vendors within your account.
Vendors from Shopify come into Inventory Planner as brands. When Shopify says 'vendor' they mean the name the product is sold under. When Inventory Planner says 'vendor' we mean supplier.
If each of your brands is sold by a unique vendor, then you can associate that information for quick vendor set up here in IP.
As long as you have some variants without an assigned vendor, then you will continue to see 'unknown' vendor. Unknown vendor cannot be re-named to your supplier. You will need to create a new vendor and assign variants to that new vendor.