Wall-Modeled Large Eddy Simulation (WMLES) has been used to study a subsonic vertical manifolds (VMs) in terms of maldistribution, i.e., how much the flow splitting deviates from an equal flow distribution between the outlets. The analyzed configuration is characterized by a wide-angle plane diffuser and by four outlets and it has been studied at high Reynolds number (, with , where is the friction velocity at the inlet, is the hydraulic diameter with A the cross-sectional area at the inlet and P the perimeter, ν is the kinematic viscosity). In the basic configuration, a jet flow develops in the diffuser with two stable flow separation regions at the inclined walls, which prevent an equal flow distribution at the outlets, and determine a maldistribution around , where ε is a parameter that quantifies the flow rate deviation from an equal distribution. To increase the equal flow distribution between the outlets, guide vanes have been used. A conceptual model to reduce the maldistribution has been developed using the momentum and the mechanical energy conservation laws. The model uses as main parameter the relative distance between the guide vanes, and it allows to minimize ε. Taking advantage of this method, the maldistribution has been reduced from , for the case of equally distributed guide vanes, to in the optimized configuration. The methodology is of general use also for hydraulic systems.