When voltage sags occur in power grids, they adversely affect the normal operation of grid-connected inverters and microgrid stability. Virtual Synchronous Generators (VSGs) exhibit transient stability issues analogous to synchronous generators during such disturbances. To address these challenges, this paper proposes a variable impedance-based stability optimization strategy for VSGs. By deploying virtual impedance control, the system maintains operational stability within defined voltage sag ranges. Through mathematical modeling of the VSG system, the stability boundaries under voltage sag conditions are analytically established. The activation of virtual impedance enables VSGs operating outside these boundaries to return to the stable region. Rigorous simulation studies conducted in MATLAB/Simulink on a single VSG unit validate the accuracy of the theoretical analysis and demonstrate the effectiveness of the proposed control strategy.