I think in order to maintain a good relationship with your vendor you should make him understand what product do you want from him. One should know what products the vendor makes other than your product so if at all we need another product similar to his other products we can ask him for the same to strengthen the ties with the vendor.
Also, communication regarding the time of delivery should be clear also the payment plan should be set already and we as a company should follow it so as to maintain the relationship with the vendor.