An optical switch using polymeric waveguides and ferroelectric liquid crystals (FLC) has been modelled by using a 3D vectorial beam propagation method (BPM) at the wavelength of 1.55 mum used in optical communications. The switch structure is a vertical directional coupler consisting of a surface stabilised FLC (SSFLC) embedded between two polymeric optical waveguides and ITO layers as electrodes. A polymeric buffer layer has been introduced to reduce losses due to ITO absorption. Polymeric waveguides and buffers used in our analysis can be obtained by different compositions of P(PFS-GMA) (poly(pentafluorostyrene-coglycidyl metbacrylate)). Several devices have been modelled with different values of the refractive indexes of the individual layers. Simulations show the benefits in terms of loss reduction due to the presence of the buffer. An optimised device presents an extinction ratio of more than 50 dB and losses lower than 1 dB with a coupling length of only 174 mum.