The mass transport system in vanadium redox flow batteries (VRFBs) is very complex, which makes it difficult to predict the cycling performance and analyze the characteristics of VRFBs. In particular, ions and water move through the membrane by various transport mechanisms such as diffusion, convection, electro-osmosis drag, and osmosis accompanied by side reactions. This complex transport system causes an imbalance in the electrolyte volume and concentration difference between the anolyte and catholyte tanks during charge/discharge cycling. As the performance of a VRFB is strongly affected by the electrolyte concentration, predicting the volume and concentration of the electrolyte is crucial to predict the performance of a VRFB and plan a rebalancing strategy for it. This study aims to accurately predict the cycling performance and efficiencies (coulomb, voltaic, and energy efficiency) of a VRFB by conducting a computational simulation that considers the electrolyte volume change, owing to the complex mass-transport system in a VRFB, for the first time. The simulation result shows that the diffusion of water and electro-osmosis of proton for an internal circuit have a dominant influence on the electrolyte volume change during the cycling process. It is observed that the electrolyte volume change is mainly caused by water diffusion in the initial cycles. Thereafter, it is found that osmosis predominantly influences the electrolyte volume change in a VRFB. The cycling performance and efficiency are calculated and validated with experimental data, which confirms the high fidelity of the model.