https://www.oracle.com/database/technologies/datawarehouse-bigdata/adb-faqs.html

VCN, it must first be empty and have no related resources or attached gateways To delete a VCN's subnets, they must first be empty.
Note: When you create one of the preceding resources, you specify a VCN and subnet for it. The relevant service creates at least one VNIC in the subnet and attaches the VNIC to the resource. The service manages the VNICs on your behalf, so they are not readily apparent to you in the Console. The VNIC enables the resource to communicate with other resources over the network. Although this documentation commonly talks about the resource itself being in the subnet, it's actually the resource's attached VNIC.
If the subnet is not empty, you instead get an error indicating that there are still resources that you must delete first. The error includes the OCID of a VNIC that is in the subnet (there could be more, but the error returns only a single VNIC's OCID).
You can use the Oracle Cloud Infrastructure command line interface (CLI) or another SDK or client to call the GetVnic operation with the VNIC OCID. The response includes the VNIC's display name. Depending on the type of parent resource, the display name can indicate which parent resource the VNIC belongs to. You can then delete that parent resource, or you can contact your administrator to determine who owns the resource. When the VNIC's parent resource is deleted, the attached VNIC is also deleted from the subnet. If there are remaining VNICs in the subnet, repeat the process of determining and deleting each parent resource until the subnet is empty. Then you can delete the subnet.
For example, if you're using the CLI, use this command to get information about the VNIC.
oci network vnic get --vnic-id <VNIC_OCID

Explanation
Oracle Cloud Infrastructure Vault lets you centrally manage the encryption keys that protect your data and the secret credentials that you use to securely access resources. You can use the Vault service to create and manage the following resources:
Vaults
Keys
Secrets
Vaults securely store master encryption keys and secrets that you might otherwise store in configuration files or in code.
The Vault service lets you create vaults in your tenancy as containers for encryption keys and secrets. If needed, a virtual private vault provides you with a dedicated partition in a hardware security module (HSM), offering a level of storage isolation for encryption keys that's effectively equivalent to a virtual independent HSM.