  We present measurements showing how disorder determines the dynamics of a
vortex lattice. Using superconductor-normal-superconductor (SNS) arrays placed
in finite magnetic fields, disorder is introduced by shifting the field away
from values where the vortex lattice is commensurate with the SNS array. By
applying a current to drive vortex motion, we observe a two-step transition at
incommensurate fields: first from pinned vortices to lattice defect motion, and
then from individual defect to bulk vortex lattice flow. This behavior is
consistent with a polycrystalline vortex structure where defects form on the
edges of crystalline domains. This demonstrates that a disordered, interacting
vortex system can favor interface physics rather than a quasiperiodic regime.
