An underground utility vault replacement method and system which create an inner structural envelope is shown to protect workers and equipment. It restores structural integrity and safety to the area within a decaying underground vault, without major disruptions above and/or below the ground. The system includes a plurality of structural walls, ceiling panels and structural beams and floor track. All structural elements are sized in width/height/thickness and methodically inserted through the existing manhole and maneuvered in place. Wall panels are attached to the floor by way of a secure track. Wall and ceiling panels are connected by way of a tongue and groove connection and an adhesive.