Sorry, I was absent for a while. Did you select the PPM app in the VESC Software and write the setting to the VESC? Did you disconnect the VESC from the computer, when you tested the PPM/RC input, or did you dectivate the keyboard controls in the VESC software? Concurrent inputs block each other or lead to jerky motor behaviour. I am on vacation right now, so I do not have the equipment right now, but I wrote about the VESC setup elsewhere on this forum before.
It is possible to monitor the PPM/RC input the VESC is receiving without actually running the motors. This is also useful to calibrate the VESCs to the pixhawk output. Sometimes there is quite a difference between what the pixhawk says it is putting out and what the VESC is receiving.
Here is the thread where I wrote about the VESC settings and calibration: