As suggested by @geofrancis, you need to try and find the source of your problem. Ruling out different parts of the system as source of the problem is the way to do it. So you need to use something to test if the VESC receives PWM signals. You can do this with a servo tester, a RC receiver that has outputs for individual channels, or a microcontroller like an arduino.
To test the outputs of the Pixhawk, you can use a servo (the servo needs power, so you need a 5V supply) or a multimeter with the ability to read frequencies. You can also try to set the Pixhawk output to passthrough or a servo setting like groundsteering, so arming and safety switch settings do not disable the outputs. I would probably do that anyway while searching for the error.