User blog:Sentry 616/An Island of Safety in a Sea of Armageddon.

 I think the vault may be intended for worst-case scenarios. Housing refugees from Vale. Mounting a do or die counter-attack. Acting as Beacon's final fallback position.

 Regardless of its intended purpose(s) I think it would serve well these several emergency roles. It is very deep, there appears to be a lot of room, and Grimm can't use elevators.