In order to address the oscillation issues induced by the increase in system transmission capacity and the reduction of grid-forming wind turbine capacity share in grid-forming and grid-following hybrid wind farms delivered via VSC DC transmission systems, a overall state-space model of the system is established. Based on this model, root locus and modal analysis methods are employed to identify the dominant contributing factors to system oscillations under different operating conditions. The analysis results indicate that system oscillations are mainly influenced by the control variables associated with the voltage outer loops of the grid-forming wind turbines and the VSC DC converter stations. On this basis, a control strategy introducing virtual impedance into the voltage outer loops of the grid-forming wind turbines and the VSC DC converter stations is proposed to improve the system impedance characteristics and suppress oscillations. Furthermore, the feasible parameter range for the virtual impedance is determined through root locus analysis. Finally, simulation results validate that the proposed virtual impedance control method can effectively enhance system performance, thereby confirming the feasibility and effectiveness of the proposed strategy.