Experimental results for the temperature-dependence of the second moment of the absorption line shape of the A- and C-bands of Sn^<2+> in KI, KBr, RbBr, and NaCl are explained satisfactorily by a model that includes a tetragonal perturbation of the O_h crystalline field due to a charge-compensating cation vacancy. The strength of the tetfagonal field perturbation is of the same order of magnitude as that of the electron-lattice interaction (dynamical Jahn-Teller effect).