The Nuclear Regulatory Commission said the Vermont Yankee nuclear power plant violated a federal security regulation this past April. The NRC, which investigated the undisclosed violation, said it involved ''an armed responder,'' or security guard. The Rutland Herald reported that the federal agency called it a ''non-cited violation'' but would not provide other details. The NRC, which made the announcement on Friday, said ''disclosure to unauthorized individuals could present a security vulnerability.'' (AP)