"Secure Vault" is being used here in the context of tokenization, which almost always means "the Vault that someone else (e.g., your payment processor) keeps the cards in, and you only ever deal with the tokens that correspond to items in the vault."
You can, of course, implement a "vault" yourself - that means a data store for PAN data which meets all the criterion of the PCI DSS such as encryption, authentication, access control, logging, etc. etc. Read the DSS, implement what it says, and do it well enough to pass an audit - voila, you're done!
But, again, "vault" is used to identify the data store backing tokens. If you're storing cards in your vault right next to the system where you're storing and using the tokens, you haven't gained anything; you still have to do all the work of protecting the PAN data. If you're not using tokens, it's not a vault, it's just a database with PAN data in PCI scope and subject to the DSS.
The purpose of tokenization is to limit the burden of the DSS. If your processor stores the PAN data, and only gives you the tokens, then you do not need to store and maintain them to the same level of compliance that you would have to if you stored the PAN data. "Vault" is just a term that describes this separation between the sensitive storage tier and the end-user (you, the merchant) tier (that is, the tokens).