A work vehicle includes a work implement. A control system for the work vehicle includes an input device and a controller. The controller is configured to communicate with the input device, receive an input signal indicating an input operation by an operator from the input device, acquire vehicle information including a position of the work vehicle when the input signal is received, and orientation information of the work vehicle when the input signal is received, and determine a target design surface indicating a target trajectory of the work implement based on the vehicle information and the orientation information when the input signal is received.