This article assumes that you have set up a minimum vault: https://dev.to/frederickollinger/production-hashicorp-vault-minimal-configuration-485a
A secrets engine is a place to keep secrets. There are a number secrets backend for various reasons. For example, there is a backend for SSL certificates and one of SSH certificates.
This is a one time operation that the admin must do when setting up a new Vault instance:
vault secrets enable ssh
Assuming we have an ssh key named fileserver.pem, put it into the vault like so:
vault write ssh/keys/fileserver email@example.com
That's all that's needed to get a key into Vault.
Sometimes things are easier than we imagine if we only have a good guide.