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Virtual Synchronous Machines (VSMs)-based high voltage DC (HVDC) systems enhance the inertia of the power system. However, the dynamic interactions between the VSM-based rectifier station, the inverter station, and the grid could induce the system oscillation, which has been investigated in this paper. At first, the hybrid AC/DC impedance models of the VSMs considering the coupling between the AC and DC dynamics are established. Then, the relationships between the DC impedance, dq-frame impedance, and the hybrid AC/DC impedance are presented. It is found that the DC impedance of the VSM-based inverter station and the d-d channel impedance of the dq-frame impedance of the rectifier station behave as negative resistors in the low-frequency range. Moreover, a fivedimension impedance stability criterion based on the hybrid AC/DC impedance and generalized inverse Nyquist criterion is proposed to assess the system stability. The analysis results show that the low-frequency oscillation occurs when the grid SCR of the rectifier station is small.