There was still one problem. If he was fixing the Vanish Cabinet here, that would mean that Voldemort plans to use it at some point. While various possibilities of usage that could be, the only one that popped into my head was having his people infiltrate the school. To what ends? Would he really be willing to send his Death Eaters into the school? The very school that I and Dumbledore were in? It was a veritable meat grinder. If I activate the defense system, there was little chance of any of them escaping. 