Dynamic stall is the main reason for the low power efficiency of vertical axis wind turbines (VAWTs) at low tip speed ratios, where the utilizable wind kinetic energy is also high. However, the determining factors behind VAWT dynamic stall have not been adequately investigated. Therefore, we carefully investigated VAWT dynamic stall under the different Reynolds numbers (Re) and reduced frequencies (k). The unsteady flow characteristics are identified using transitional URANS simulations. Although the blade undergoes significant vortex movements during each revolution, the output power is primarily determined by the aerodynamic responses close to the onset of dynamic stall. Increasing Re and k can impact dynamic stall behaviors significantly and also improve VAWT performance effectively. As both Re and k increase, the onset of dynamic stall is progressively postponed to a higher angle of attack, effectively suppressing the separated flow. Dynamic stall behaviors are therefore changed from abrupt stall to moderate stall with attenuated laminar separation bubble (LSB) bursting and dynamic stall vortex (DSV) shedding. An increase in Re hardens the LSB and prevents the DSV formation due to strong inertial forces, particularly at low Re. On the other hand, increasing k slows the flow-field transitions and delays the onset of LSB bursting and DSV shedding, particularly at high k.