I found out this hard way and had to look in VehicleMovement source code (same goes for CharacterMovement). There is explicit checks in this component which will ignore doing any movement if controller is not set for the owner (i.e. pawn). The easiest way to deal with this is set followings for the pawn:
- Auto Possess Player: Disabled
- Auto Possess AI: Placed in World or Spawned
- AI Controller Class: AIController
Above will cause pawn not getting auto-possed by player controller but still you will be able to set throttle etc on VehicleMovement and pawn will respond. Note that this would not work is you set Auto Possess AI: Placed in World which is default.