A vehicle steering system is provided with a primary steering system, a backup steering system and a control system that switches from the primary steering system to the backup steering system when a defect is determined in the primary steering system such as a defeat in a steering reaction force actuator. The vehicle steering system has a pair of clutches that mechanically disconnect the steering wheel from the steerable vehicle wheels in normal situations, and connect them when the steering reaction force actuator is determined to be defective. The control system has a steering malfunction detection section that detects a malfunction in the steering reaction force actuator, and a steering control limiting section that limits the control commands to the steering gear unit actuator from when the steering reaction force actuator is determined to be malfunctioning to when both the clutches are connected.