I'm trying to have a better understand of Secrets block. Say I have a k8s cluster at one of my customers, Inside there the k8s cluster there is a hashicorp vault service (not cloud). Can i set up a secret who's secret- provider is this vault k8s service and non of those secrets are actually kept in prefect cloud ?

If I’m understanding you correctly, then yes, that’s possible. We have a different, vault specific, block that you can use: https://github.com/pbchekin/prefect-vault Essentially, instead of telling your secret to Prefect, you tell Prefect where to find the secret you want in Vault.