Hi,
Regarding the restriction to the main vendor, you can use the "vendor" restriction setting in your shipping methods to restrict them to either a particular vendor, or to the main vendor.
So I suppose you'll want to have 3 manual shipping methods, restricted to the main vendor, one for each shipping cost, and then enter the corresponding amount either in the "price" field if the shipping cost is global to all the products shipped in the cart, or in the "shipping price per product" field if the cost is per product being shipped.