In this work we propose and experimentally validate a relaxation spiking oscillator architecture with ultra-high tuning range (higher than 400%, from 5KHz to more than 25kHz when the control voltage is varied from 2.5 to 5V) that exploits the reversible metal-insulator transition in 2-terminal Vanadium Dioxide (VO2) thin film devices loaded to a MOSFET common source amplifier. We propose and validate an analytical model that connects key output signal metrics (frequency and amplitude) to the intrinsic properties of the phase-change VO2 device employed (switching thresholds, hysteresis and electrical resistance in on- and off- states). We show that the proposed analytical model of the switching dynamics enables us to accurately simulate and predict the oscillation waveform based solely on VO2 DC electrical characteristics. Finally, we report two experiments of sensing RF and optical power with VO2 devices using the spiking oscillator as a readout circuit, showing highly linear responses: (a) for the RF power sensing we report a sensitivity of 4.64 Hz/dBm in the GHz range, and, (b) for the optical power sensing, we report sensitivities as high as 4.23 Hz/mW in the range of 500 to 700 nm (visible optical spectrum).