This letter proposes an observer-based formation tracking control approach for multi-agent velocity-controlled vehicles under the assumption that either relative or global position measurements are unavailable for all the vehicles. It is assumed that only some vehicles (at least one) have access to their own global position, and all vehicles are equipped with sensors capable of sensing the bearings relative to neighboring vehicles. Each vehicle estimates its global position using relative bearing measurements and estimates of neighboring vehicles received over a communications network. Then, a distributed output-feedback observer-based controller is designed relying on bearing measurements and the estimated global positions. In contrast with the literature on bearing-based localization and control, we relax the common assumption of so-called bearing rigidity, and, in addition, we do not assume that the interconnections are constant. To the best of our knowledge, the bearing-based localization-and-tracking control problem under such assumptions remains open. In support of our theoretical findings, some simulation results are presented to illustrate the performance of the proposed observer-based tracking controllers.