I believe the Vault is an alternate dimension of time which the Vex control, correct me if I'm wrong. The Aegis is the only reason we aren't easily eliminated by the Vex (as they are infinitely powerful in the Vault) because it binds us to reality.
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You are correct, but the Aegis was created by Kabr sacrificing himself. This could be interpreted as his creating a "glitch" in the system that allows for victories.