How to check constraints on two tables without primary key -foreign

Hello
I have two tables in my sql database one is "vendor" and other "purchase_order" they both have one common attribute "vendor_name" my problem is that i want vendor name in my "purchase_order" table to be same as that in "vendor" table but vendor_name is not a primary key in "vendor" table so i cannot use primary key_foreign_key reference.
Could anyone help me what should I use?
Thanks
