  The dynamics of dislocations in a two-dimensional vortex lattice is studied
in the presence of a pinning potential and a transport current. In a vortex
lattice drifting with velocity $v$ a glide velocity $V_d$ of the dislocation
with respect to the vortex lattice is found to decay like $V_d \sim v^{-4}$ for
large drive. From this result the velocity for the crossover between a regime
of coherent elastic motion and a regime of incoherent plastic motion of
vortices is estimated.
