  We show that annihilation dynamics between oppositely charged optical vortex
pairs can be manipulated by the initial size of the vortex cores, consistent
with hydrodynamics. When sufficiently close together, vortices with strongly
overlapped cores annihilate more quickly than vortices with smaller cores that
must wait for diffraction to cause meaningful core overlap. Numerical
simulations and experimental measurements for vortices with hyperbolic tangent
cores of various initial sizes show that hydrodynamics governs their motion,
and reveal distinct phases of vortex recombination; decreasing the core size of
an annihilating pair can prevent the annihilation event.
