  Current-voltage characteristics of a single-electron transistor with a
vibrating quantum dot were calculated assuming vibrons to be in a coherent
(non-equilibrium) state. For a large amplitude of quantum dot oscillations we
predict strong suppression of conductance and the lifting of polaronic blockade
by bias voltage in the form of steps in $I-V$ curves. The height of the steps
differs from the prediction of the Franck-Condon theory (valid for equilibrated
vibrons) and the current saturates at lower voltages then for the case, when
vibrons are in equilibrium state.
