The effect of the mass of a 2D elastically-mounted circular cylinder in cross-flow on its vortex-induced vibrations and on the related vortex shedding lift forces is analyzed via a single-degree-of-freedom multi-frequency model (sdof-mf). The mechanical system in question is characterized by low mass ratio, low structural damping and Reynolds number of order \(10^4\). The proposed sdof-mf model relies on the decomposition of the total hydrodynamic force in a inertia/drag force, conventionally associated with the cylinder motion in still fluid, and an additional lift force associated to pure vortex shedding. The lift force is assumed to be composed by not-Fourier-dependent harmonics; this constitutes the key point of the proposed sdof-mf model. The parameters of this model are determined via a parameter identification method based, in this case, on VIV data obtained via CFD. The simulations are carried out changing systematically the values of the mass ratio, within the range of engineering practice, and covering a wide range of flow regimes including lock-in conditions. The results from the application of the sdof-mf model highlight the large influence of the mass ratio on the response of the cylinder and on the vortex shedding lift force. The effects are clearly visible on the maximum amplitude at lock-in, on the range of incident flow velocity over which synchronization occurs, on ultra/sub harmonic behavior and phase lag of the cylinder motion, and finally on the magnitude and harmonic content of the lift force induced by pure vortex shedding.