A vendor agreement is a legal agreement that clearly states the provisions and conditions of the work to be performed by a contractor. The key points to be included in a vendor agreement include date, time and location where the products will be delivered or the services must be provided. A vendor agreement must always be accompanied with a Statement of Work (SOW). A vendor will not start work without an acceptable form of SOW. A vendor agreement becomes valid and enforceable when the customer and vendor signs the agreement in original.