As a follow-up of a previously published article on the contribution of tropical waves, this study explores the evolution of the mid-tropospheric mesoscale cyclonic vortex (MV) during the formation of Typhoon Megi (2010) with a successful cloud-resolving simulation. It is found that the formation and intensification of the MV were related to the deep convection and subsequent stratiform precipitation, while the weakening of the MV was related to the shallow convection. Both the upward transport of vorticity related to the deep convection and the horizontal convergence associated with the stratiform precipitation contributed to the formation and intensification of the MV. Even though the latter was dominant, the former could not be ignored, especially in the early stage of the MV. The MV played dual roles in the formation of Megi. On the one hand, the formation and intensification of MV were primarily associated with the stratiform precipitation, which induced the low-level divergence inhibiting the spin-up of the near-surface cyclonic circulation. On the other hand, the coupled low-level cold core under the MV benefited the accumulation of the convective available potential energy (CAPE), which was favorable for the convective activity. A sensitivity experiment with the evaporative cooling ted off indicated that the development of the MV retarded the genesis process of Megi.