The value VPVehicleController.isCollisionEnter is used when subscribing to VPVehicleController.onRawImpact. Otherwise its value is undetermined. onRawImpact simply forwards the events OnCollisionEnter and OnCollisionStay generated by the rigidbody.
VPVehicleController.onImpact is different. This event compiles the positions and velocities of a number the contact points along several OnCollisionEnter/OnCollisionStay events, then raises timed onImpact events from FixedUpdate. These "managed" impacts are much more convenient for audio effects, visual effects, damage, etc than using the raw collision events. This is especially relevant on high physics rates, where 100's of OnCollision events may happen in a very short time.