Phase-change materials, such as vanadium dioxide (VO₂), exhibit high contrast in their optical properties upon transitions between their insulator and metallic phases, providing an interesting tool to design active devices. A cavity structure consisting of a VO₂ ring within a nanodisk is proposed to control the propagation of light through a metal–insulator–metal plasmonic waveguide. Numerical simulations indicate that the transmission of the designed structure is higher than 0.56 in the wavelength range of 1500 to 1600 nm in the insulating phase of the VO₂ rings. However, transition of the VO₂ rings to the metallic phase considerably increases their absorption coefficient. Then the transmission of the structure falls to lower than 0.0034 in the 1500- to 1600-nm wavelength range. Therefore, the achieved extinction ratio is higher than 22.1 dB in this wavelength range. We demonstrate that the central wavelength of the transmission band of the structure can be tuned by changing the geometrical parameters of the VO₂ ring. The influence of partial phase transition on the transmission level is also studied by effective medium theory.