Planarity of cis- and trans-3-chlorostyrene is discussed in the light of new data from infrared/Raman and inelastic neutron scattering (INS) spectroscopies and theoretical calculations. Molecular structures have been optimized at different levels of theory, ab initio and DFT, with an assortment of basis sets, 6-31G*, 6-311G** and 6-311++G** concluding that Hartree-Fock and MP2 results predict a non-planar structure while DFT predicts a planar geometry regardless of the basis set. Vibrational spectra have been calculated and compared with experimental, data as obtained from IR, Raman and, for the first time, Inelastic Neutron Scattering (INS) providing evidence for the planarity of this system in its ground electronic state. Molecular force fields for the cis and trans conformers of 3-chlorostyrene using the scaled and refinement formalisms are reported. For easier visualisation the valence force constants have also been converted to a so-called pure vibrational force field. These results together with the barrier height calculations to vinyl internal rotation allows conclusions about the extent of localisation of the vinyl double bond.