This work presents the geometrically nonlinear free and forced vibration analysis of a simply supported sandwich plate with a soft viscoelastic core. Damping is included by modelling the viscoelastic core as a Voigt-Kelvin solid. The nonlinearity is included in terms of von Karman strains and Reddy's third order shear deformation theory is used to model the deformations. Governing equations are obtained by Lagrange's formulation. A combination of Homotopy perturbation method with Struble's technique is used to solve the strongly nonlinear system for free and forced vibrations for a point load. The variations of nonlinear amplitude and phase difference with forcing frequency are obtained and the influence of layer thickness, length to thickness ratio, applied force and the damping parameter on the damped response is studied. © CCM 2020 - 18th European Conference on Composite Materials. All rights reserved.