According to the conversation at http://forum.ansible.com/t/ansible-inventory-as-json-and-inline-vaulted-data-does-not-work-works-with-yaml-inventory/2909
true .yaml inventories permit vaulted values, so that should work for your requirements.
You’ll need to convert your inventory from .ini format to .yml, and vault your password with “ansible-vault encrypt_string…”,