Vertical Axis Wind Turbines (VAWTs) generally have lower performance compared to Horizontal Axis Wind Turbines (HAWTs), due to the more complex and unsteady turbulence flow conditions around VAWTs. To compete with large-scale HAWTs, VAWT requires further improvement of its performance, particularly in terms of power coefficient and self-starting ability. In this study, numerical simulations have been carried out to compare the effectiveness of performance enhancement of VAWTs across different Tip Speed Ratio (TSR) regimes between a dynamic stall control device (Gurney flap (GF)) and a flow augmentation device (straight upstream deflector (SUD)). Additionally, a configuration combining both GF and SUD was also investigated. The computational fluid dynamics (CFD) solution using a hybrid Reynolds averaged Navier-Stokes and large-eddy simulation (RANS-LES) applying stress-blended eddy simulation (SBES) with transition shear-stress transport (TSST) turbulence model was adopted. A baseline VAWT configuration and its variants were evaluated to consider the rotating blade and blade-to-blade interaction effects. The results indicated that a standalone GF produces higher averaged power coefficient (𝐶𝐶−𝐶𝐶𝐶) improvement of VAWT at low and medium TSR regimes. Both GF and SUD devices have exhibited the reduced effectiveness as TSR increased, with GF having a higher loss of its ability to enhance 𝐶𝐶−𝐶𝐶𝐶 at high TSR regime while compared to SUD. Therefore, this study suggests that GF is more suitable for enhancing VAWT performance at low and medium TSR regimes where the dynamic stall at these two regimes is dominant. Conversely, SUD is more effective for improving VAWT performance at high TSR regime, where the flow is relatively stable due to weaker dynamic stall. However, the devices by combining GF and SUD does not show further improvement of 𝐶𝐶−𝐶𝐶𝐶 compared to their standalone devices. Such a combination did generate stronger vortex, primarily due to SUD, but GF lost its ability to suppress or delay the dynamic stall, introducing higher drag force and reducing performance.