Variability and uncertainty are intrinsic characteristics of power systems. On the other hand, high penetration of Distributed Energy Resources (DERs), especially variable output generations such as winds and photovoltaic systems, exceeds the expected variability and uncertainty of a power system. In this situation, the integration of a DER portfolio can be done following the concept of aggregation of these resources. If several DER units are linked together and are operated as one unit this concept is often called a Virtual Power Plant (VPP). The aim of this paper is to carry out an optimal dispatch that the operator of a VPP is able to maximize its profit in consideration of the relevant technical and economical constraints as well as some existing uncertainties. The profit maximization problem also considers demand side bidding for dispatchable loads, Renewable Obligation Certificate (ROC) for renewable based Distributed Generations (DGs) and the cost related to Use of System (UoS) charges to Distribution System Operator (DSO). The simulation results for a typical system demonstrate the effectiveness of the method.