Vault: a secure place to store valuables.
He likes walking through the collection during the day. It doesn't make it safer. There's nothing he can add with his vigilance to the tens of millions of dollars in security systems, the self-contained architecture of the building, or the geographic isolation that is also a form of defense. It simply makes _him_ feel better to show himself that the collection exists. To know that so many extinct species are only almost gone.
Besides, there are no windows in the storage rooms. He cannot bear looking at the green artic fields outside.
Vault: a burial chamber.
A reproductively viable tissue sample isn't a species. A population of forced-diversity clones isn't a resurrection but an engineering project. That's not nothing, he tells himself in the sleepless nights when the sun never sets and the promises of a better future ring as hollow as the people making them. He stays outside the collection rooms to avoid disturbing the dead.
Vault: a self-supported arched ceiling often covering a sacred space.
He doesn't look at the news. When supply crews come he doesn't talk much and he doesn't ask questions. There's a strange air to them, between bewildered and haunted, and he doesn't know if it's a reaction to the vault or to everything else. They might not know it themselves; the vault keeps things from changing by keeping them from the world, including himself.
He understands from things unsaid that his vault is the last one although not the reason why. They carry weapons but don't offer them. He would have refused anyway.
Vault: the space within the cranium occupied by the brain.
There's untapped knowledge in the collection. Biology is still largely a trial-and-error affair and some of Nature's trials are only recorded in the samples in the vault. It's a vague form of potential knowledge. He thinks if anybody took it seriously the vault would be elsewhere and the samples wouldn't be stored. It's not a world that keeps anything of value unexploited. Still, when he sits among the samples he thinks things he had never thought before. That's a form of knowledge too.
Vault: (figuratively) anything resembling an architectural vault, for example the sky.
The supply crew finds the vault silent, storage rooms empty, caretaker gone. No signs of violence. Nothing in the logs but the automated remote updates. The search is driven by annoyance as much as by fear or a sense of loss and nobody complains when it's called off.