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Vortices or merons are the relevant topological solitons in a two-dimensional isotropic Heisenberg antiferromagnet immersed in a uniform magnetic field. The dynamics of such solitons is studied numerically within the discrete spin model as well as analytically within a continuum approximation based on a suitable extension of the nonlinear model. Vortex dynamics is affected rather profoundly by the applied field and acquires the characteristic features of the Hall effect of electrodynamics or the Magnus effect of fluid dynamics. In particular, a single vortex is always spontaneously pinned, two like vortices form a rotating bound state, and a vortex-antivortex pair undergoes Kelvin motion.