Secret Key in 1Password Vault?

It's secure. It's similar to physical vaults and physical vault keys where you might keep currently unused additional vault keys in the vault itself. It's only possible to access it, if you already authenticated yourself and signed in.

But it's important to store it additionally in some place external to 1Password, because you need it if you add a new device and at the same time don't have a signed in device.

For disaster recovery, print it, write the master password on it and in case you activated mfa for your 1Password account, also add a printed version of the mfa QR code. This enables you to get access to your 1Password account again even if you lose access to every computer device.

In general, make sure you store it in a place you still have access to, even if you sign out from every 1Password client and browser session, to avoid a circular dependency.