Absorption and magnetic circular dichroism (MCD) spectra of band C for the Sn2+ centre in alkali halide crystals have been calculated by means of the Monte Carlo integration method. The calculated spectra are compared with the asymmetric absorption and inversion-asymmetric MCD spectra observed in KCl:Sn2+ crystals. A good fit to the observation is obtained taking into account both the quadratic electron-lattice interaction with the T2g vibrational mode and the charge-compensating vacancy in addition to the linear electron-lattice interaction. It is shown that, although the quadratic interaction gives rise to an asymmetric MCD lineshape, the vacancy is important to derive the spectra which agree with the observations for both the absorption and the MCD.