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Actively tunable metal-insulator-metal waveguides that employ vanadium dioxide films as the active medium are analyzed numerically. Vanadium dioxide exhibits strong contrast between the optical properties of its insulating and metallic phases. In particular, the large optical absorption in the metallic phase makes it straightforward to implement broadband attenuation modulators and switches, but this strong loss can also complicate the design of other types of devices. We present a plasmonic waveguide that functions as an index modulator with Δn > 20% at λ_0 = 1550nm (0.80 eV), by using a thin active layer to strike a balance between maximizing index contrast while mitigating attenuation. A second device is configured as a band-stop absorption modulator, taking advantage of symmetry to selectively suppress the TM1 and TM3 modes, with relatively minimal attenuation of the TM0 and TM2 modes.