  We derive an approximate dynamical equation for the form-factor of the
ghost-gluon vertex that contributes to the Schwinger-Dyson equation of the
ghost dressing function in the Landau gauge. In particular, we consider the
"one-loop dressed" approximation of the corresponding equation governing the
evolution of the ghost-gluon vertex, using fully dressed propagators and
tree-level vertices in the relevant diagrams. Within this approximation, we
then compute the aforementioned form factor for two special kinematic
configurations, namely the soft gluon limit, in which the momentum carried by
the gluon leg is zero, and the soft ghost limit, where the momentum of the
anti-ghost leg vanishes. The results obtained display a considerable departure
from the tree-level value, and are in rather good agreement with available
lattice data. We next solve numerically the coupled system formed by the
equation of the ghost dressing function and that of the the vertex form factor,
in the soft ghost limit. Our results demonstrate clearly that the
nonperturbative contribution from the ghost-gluon vertex accounts for the
missing strength in the kernel of the ghost equation, and allows for an
impressive coincidence with the lattice results, without the need to
artificially enhance the coupling constant of the theory.
