  We use the recently proposed scattering states numerical renormalization
group (SNRG) approach to calculate $I(V)$ and the differential conductance
through a single molecular level coupled to a local molecular phonon. We also
discuss the equilibrium physics of the model and demonstrate that the
low-energy Hamiltonian is given by an effective interacting resonant level
model. From the NRG level flow, we directly extract the effective charge
transfer scale $\Gamma_{\rm eff}$ and the dynamically induced capacitive
coupling $U_{\rm eff}$ between the molecular level and the lead electrons which
turns out to be proportional to the polaronic energy shift $E_p$ for the
regimes investigated here. The equilibrium spectral functions for the different
parameter regimes are discussed. The additional phonon peaks at multiples of
the phonon frequency $\w_0$ correspond to additional maxima in the differential
conductance. Non-equilibrium effects, however, lead to significant deviations
between a symmetric junction and a junction in the tunnel regime. The
suppression of the current for asymmetric junctions with increasing
electron-phonon coupling, the hallmark of the Franck-Condon blockade, is
discussed with a simple framework of a combination of (i) polaronic level
shifts and (ii) the effective charge transfer scale $\Gamma_{\rm eff}$.
