The Angle of Attack (AOA) of the Vertical Axis Wind Turbines (VAWTs) blades has a dominant role in the generation of the aerodynamic forces and the power generation of the turbine. However, there is a significant uncertainty in determining the blade AOAs during operation due to the very complex flow structures and this limits the turbine design optimization. The paper proposes a fast and accurate method for the calculation of the constantly changing AOA based on the velocity flow field data at two reference points upstream the turbine blades. The new method could be used to calculate and store the AOA data during the CFD simulations without the need for extensive post-processing for efficient turbine aerodynamic analysis and optimisation. Several single reference-points and pair of reference-points criteria are used to select the most appropriate locations of the two reference points to calculate the AOA and It is found that using the flow data from the two reference points at the locations 0.5 aerofoil chord length upstream and 1 chord away from each side of the aerofoil can give most accurate estimation across a range of tested AOAs. Based on the proposed AOA estimation method, the performance of a fixed pitch and the sinusoidal variable pitch VAWT configurations are analysed and compared with each other. The analysis illustrates how the sinusoidal variable pitch configuration could enhance the overall performance of the turbine by maintaining more favourable AOAs, and lift and drag distributions.