He was standing outside the Benfords' house to protect the residents and/or spy on Lloyd. He tells the other man that "Mark Benford is dead" so as to cover for Mark/ buy him the time necessary to stop the second GBO. This might have been part of a 'deal' Vogel made with whoever wanted to cause the second GBO. (Jericho?). If one takes a very close look at the facial expression of Vogels conversational partner during the GBO, he seems to like the message that Mark Benford is dead. That would underline the idea of Vogel pretending Mark's dead.