Parametrization of the decay index ndecay of the equilibrium magnetic field in a tokamak with respect to plasma geometry, βp value and profile peakedness is investigated numerically for a fundamental understanding of vertical displacement instability (VDI). A D shaped plasma with higher triangular deformation and broader profiles is more stable against VDI. For the parameters referred to, as well as those of both DIII-D and ITER, pressure can play either a stabilizing or a destabilizing role, depending on both the geometry and the profile peakedness of the plasma. A saturation of ndecay with respect to the elongation and the profile peakedness exists