A new model for the vibronic problem of luminescence of 3d³ impurity ions in the cubic environment is proposed. The approach developed is based on the numerical quantum-mechanical solution of the dynamic pseudo Jahn-Teller vibronic problem for the excited states. Spin-orbit splitting of ⁴T₂ term and its mixing with ²E as well as the vibronic interaction with the full-symmetric (A_1g) local mode is taken into account resulting in the [formula omitted] vibronic problem and [formula omitted], adiabatic problems. The band shape of the luminescence and the temperature dependence of the life-time decay emission are evaluated for Cr³⁺ ions in CdIn₂S₄:Cr single crystals. The proposed model provides a very good explanation of the experimental data.