Sandbox Vaults are copies of Production Vaults.
They can be used by an organization to develop and test configuration changes, data migrations, and integrations, without affecting the Production Vault and users.
Sandbox vaults are critical to an effective change control process. Creating a Sandbox for a new project and refreshing Sandboxes often helps an organization avoid issues and delays when deploying changes in Production.
In this case, the system admin cannot see the Sandbox Vaults option in Admin --> Deployment.
The system admin can access from a Sandbox Vault but the Sandbox Administration options are available from the Production Vault.
Access Sandbox Administration options from the Production Vault.
Vault Help Documentation: Administering Sandbox Vaults