Atheon exists outside of time, or rather the apex of all possible times. Several versions of the timelines contain Guardians that decide to enter the Vault of Glass. Several versions beat Atheon. Some enter sooner then others, but all of those that defeat Atheon defeat a valid and existing Atheon. Yet Atheon can only exist outside of time...therefore he can fall only once. That is what Time's Vengeance is. Its every Guardian that has ever shot him... shooting him at once. This is because there is only one Aegis, because the Aegis was created with Kabr's Light and Gorgon material. Atheon is dead, but he is also not dead because he needs to be alive when the future people beat him. So re-entering the Vault of Glass is just repeating the Paradox.