Who was in the network with Failsafe? My guess would be Osiris, because of the Pyramidion strike we do. In it you hear Ikora say she thought she heard a voice she hadn't heard in years. And being a disciple of Osiris she would have known his voice very well. Osiris was believed to have been obsessed with the Vex, as Toland the Shattered was with the Hive. It wouldn't be that hard to believe that Osiris downloaded his mind into the Vex network at some point. Possibly even to get a better understanding of the Vex.