The best way to do it would be to have a Vendor table, like
Estimate --< LineItem >-- Product >-- Vendor
where the vendor of a product id is denoted via a foreign key in the Product table.
This way, you could simply put a portal into Vendor on the Estimate layout.
I believe there is a way to accomplish this given the current structure, however I have just modeled your suggestion and it works. Thanks so much for the quick response!!