The Vault Admins group is a predefined group that is automatically created during the installation or upgrade of the Vault. This group has all possible permissions in the Vault, and can create and manage other users, groups, platforms, policies, safes, and accounts. However, this group is not automatically added to every safe in the Vault, but only to some system safes that are used for administrative purposes. Therefore, being a member of the Vault Admins group does not guarantee that you can grant any permission on any safe, unless you are also a member or an owner of that safe. To grant permissions on a safe, you need to have the Authorize safe members authorization on that safe, which allows you to add or remove users or groups as safe members, and assign or revoke their authorizations. Alternatively, you can use the Administrator user, which is a predefined user that is a member of the Vault Admins group, and has all possible permissions on any safe in the Vault. References:

Predefined users and groups

Safe member authorizations