I use Ansible Vault to securely store the project's secrets (e.g. API keys, default passwords, private keys, etc.) in the git repository for many of my infrastructure projects. I also like to make sure I cover everything possible in automated tests/CI, using either Jenkins or Travis CI (usually).
But this presents a conundrum: if some of your variables are encrypted with an Ansible Vault secret/passphrase, and that secret should be itself store securely... how can you avoid storing it in your CI system, where you might not be able to guarantee it's security?
The method I usually use for this case is including the Vault-encrypted vars at playbook runtime, using