The energy shift of the vibrational levels of the X and A states due to the vibronic interactions within the manifold of states is accurately determined using a diabatic representation. These vibronic shifts which measure the breakdown of the Born - Oppenheimer approach are compared with the usual estimation where only the adiabatic correction is taken into account. The adiabatic corrections lead to a systematic overestimation of the full vibronic shift for the ground state and to an underestimation for the A one. These results have been related to the interstate dynamical couplings and rationalized using perturbative arguments. Their validity is expected to be very general.