This work applies a combined approach, based on a reduced-order model (ROM) together with experiments and direct numerical simulations, to investigate the optimal efficiency of fluid-flow energy harvesting from transverse vortex-induced vibration (VIV) of a circular cylinder. High-resolution efficiency maps were predicted over wide ranges of flow reduced velocities and structural damping ratios, and the maximum efficiency and optimal settings of damping ratio and reduced velocity were then examined for different mass ratios and Reynolds numbers. Efficiencies predicted by the ROM were also validated against either experiments or direct simulations. The present work indicates that: (i) the simple ROM, with low costs, is a useful tool to estimate and optimise the energy harvesting efficiencies from VIV; (ii) the maximum efficiency is controlled by both the incoming reduced velocity and the product of mass ratio and structural damping ratio, which is similar to the maximum amplitude of VIV; (iii) the maximum efficiency at a relatively high Reynolds number (Re≈6×103) in subcritical regime is higher than that of a low Reynolds number (Re=150) in laminar regime; (iv) the energy harvesting efficiency from VIV of a circular cylinder with a low mass ratio is more robust than that with a high mass ratio. This finding suggests that the VIV harvester performs better in water than in air.