There was a discussion about VCO in this post
How to make a voltage controlled oscillator in simulation? - QSPICE - Qorvo Tech Forum
However, a VCO in general is designed to take a control voltage as input and produce a square or sine wave with a frequency controlled by that input voltage. From what you have shown above, with a clk100m input resulting in a pulse at 100kHz, are you truly seeking a VCO?