Vanadium dioxide undergoes a reversible metal-insulator phase change near 68°C. This transition is often recognized for its electronic properties; however, the concomitant change in optical properties is also attractive. One application is to exploit the optical properties within this transition region to control and tune the refractive index between its insulating and metallic states. To achieve this, we used atomic layer deposition to grow high-quality, low-temperature (≤150°C), ultrathin films of vanadium dioxide on sapphire substrates. Measurements of optical transmittance and reflectance as a function of temperature are then used to create wavelength and temperature dependent models of the complex optical refractive index. Our models create a foundation for developing vanadium dioxide as a tunable refractive index material.