  The motion of non-circular, two-dimensional vortices is shown to depend on a
new form of coupling between vortex ellipticity and the gradient of fluid
density. The approach is based on the perspective that such vortices are
projections of tilted, three-dimensional, virtual counterparts. The resulting
kinetic equation offers new insights as to how tilt and vortex velocity
co-evolve in few-body, non-equilibrium settings such as vortex pair nucleation
and annihilation. The model is developed and applied in association with
optical vortices, and optical experiments are used to verify its predictive
power. It is valid for quantum fluids and classical hydrodynamics settings as
well.
