Please sign the agreement electronically or it may delay processing. A vendor agreement in Washington is a legally binding contract between a vendor and a customer.It outlines the terms of the sale of goods or services. New to the vendor registration system? 3. Make payments in a timely manner to the vendor on behalf of eligible households for the term of this agreement. 4. We'll help you craft the best vendor contract agreement for your specific needs, and help you manage that relationship as long as you need it.