Blend membranes comprising sulfonated poly(ether ether ketone) (sPEEK), poly(vinylidene fluoride) (PVdF), and urethane acrylate non-ionomer (UAN) were prepared for application in vanadium redox flow batteries (VRFBs). The key idea of this work is to combine the high proton-conducting property of sPEEK and low vanadium permeability of PVdF by introducing the amphiphilic UAN to the immiscible blend of sPEEK and PVdF, UAN makes the two polymers miscible. The blend membranes are characterized by water uptake, dimensional change, proton conductivity, vanadium ion permeability, and cross-sectional morphology. By introducing small amounts of UAN (1-2 wt%) to the sPEEK/PVdF blend, proton conductivity increases, while vanadium permeability shows a very small change, resulting in higher proton selectivity against vanadium ion transport. Therefore, VRFBs employing the sPEEK/PVdF/UAN blend membrane exhibit significant improvements in discharge capacity and energy efficiency (EE) in comparison with those based on the sPEEK/PVdF blend and even with a Nafion 212 membrane. These results show that UAN-assisted compatible sPEEK/PVdF/UAN blend membranes can be useful for enhancing the VRFB performance.