Microgrids controlled by virtual synchronous generators (VSG) experience inrush currents and voltage offsets during transitions between grid-connected and islanded modes. These phenomena cause fluctuations in active power and frequency, thereby affecting the safe and stable operation of the system. To address this issue and achieve seamless mode switching in microgrids, the fundamental principles of VSG control and the linear active disturbance rejection control (LADRC) algorithm are investigated in this paper. The second-order LADRC model is simplified and incorporated into the active power-frequency control loop of the VSG. This replaces the traditional proportional-integral (PI) controller in the phase pre-synchronization unit. The output frequency of the VSG is taken as the output of the LADRC, and the rated frequency is taken as the input of the LADRC, while regulating both the frequency and phase simultaneously. It also reduces the impact of inrush currents and voltage offsets during grid connection. This prevents excessive inrush currents that could destabilize grid integration and enables seamless switching between grid-connected and islanded modes. MATLAB/Simulink simulation models are constructed to compare the proposed control strategy with direct grid connection without pre-synchronization and with traditional pre-synchronization control strategies. The results validate that the proposed control strategy suppresses inrush currents and voltage fluctuations more effectively, which provides more accurate power tracking, and achieves better performance in mode switching.