[en] The vibrational model is extended by introducing isospin-dependent collective coordinates, permitting a description out-of-phase neutron-proton vibrations coupled by a density-dependent symmetry energy. The restoring force is calculated microscopically using the wavefunctions of a Woods-Saxon potential and the coupling with three-phonon states is taken into account. The model is able to describe the available experimental data (energies and multipole mixing ratios) on low-lying 2⁺ states, which were observed recently in nuclei near the shell closures (¹²⁴Te, ¹⁴⁰Ba, ¹⁴²Ce and ¹⁴⁴Nd), supporting the identification of these states as isovector quadrupole vibrations and predicting such states in ^126-130 Te. (author)