This paper investigates a two-echelon single-vendor multiple-buyers supply chain model. This model formulates an under Vendor Managed Inventory (VMI) mode of operation, in order to maximizing the profit of the supply chain, based on optimizing the length of the buyer ordering period; the relationship between the demand of buyers and the sales price is also considered linearly. In this paper, for the first time, a two-echelon single-vendor multiple-buyers supply chain model is considered under multi-item and joint ordering assumptions. Also, analysis of the model is carried out with a numerical study. Furthermore, the numerical example is solved using the LINGO tool, and the sensitivity of the model, with the number of buyers and the constant ordering cost, is investigated.