Nonequillibrium pair attraction between colloidal particles, constrained to move along a circular ring by an optical vortex, is observed, when the radial confinement is sufficiently weak. This dynamical pairing causes the particles to assemble into specific configurations (limit cycles). The configuration selection depends on initial conditions, temperature and radial confinement strength. We propose a mechanism for the particle pairing, based on radial symmetry breaking, and support it by experimental and simulation results. Optical vortices containing two and four particles are further discussed, focusing on statistics and particle configurations. We show the influence of particle density in a vortex on the collective mobility of the particle assembly, and discuss the theoretical limit of infinite particle number.