  We study the wave breaking mechanism for the weakly dispersive defocusing
nonlinear Schroedinger (NLS) equation with a constant phase dark initial datum
that contains a vacuum point at the origin. We prove by means of the exact
solution to the initial value problem that, in the dispersionless limit, the
vacuum point is preserved by the dynamics until breaking occurs at a finite
critical time. In particular, both Riemann invariants experience a simultaneous
breaking at the origin. Although the initial vacuum point is no longer
preserved in the presence of a finite dispersion, the critical behaviour
manifests itself through an abrupt transition occurring around the breaking
time.
