Authorities said a Vermont trooper was justified in using deadly force on an unarmed burglary suspect who refused an order to surrender and twice took a ‘‘shooter’s stance’’ while aiming what turned out to be a cellphone at him. Attorney General William Sorrell and Lamoille County State’s Attorney Joel Page said Friday that the trooper was reasonable in his belief that he was in imminent danger of death or serious bodily injury when he fired his weapon at Jonathan Martel on April 30. Martel died of his injuries. The trooper who fired the shot, Senior Trooper Dustin Robinson, 31, is a three-year veteran who was the only person who witnessed the shooting in Cambridge.