Throttle and brake are different inputs in VPP vehicles. However, the standard input component (VPStandardInput) uses Input.GetAxis("Vertical") for both throttle and brake.
You may send different values to each input separately by writing an script that reads the proper input axes and send the values to the vehicle inputs. Check out:
https://vehiclephysics.com/advanced/databus-reference/
However, I'm not sure that the Xbox One gamepad could be read properly using Unity Input. The last time I tried both triggers were acting as a single axis. You may need to use a third-party input solution such as Rewired, or the upcoming Unity Input System (currently in preview).