Hi. If I understand your question right, you don’t want enter vault password manually when running playbook - instead of this you want take this password from file.
I’m pretty sure, that you already checked all related to it documentation, so excuse me in advance - but let me to post here link to https://docs.ansible.com/ansible/2.4/vault.html#providing-vault-passwords
I propose check this link just because new information added for 2.4 version and this probably describe our case and additional question for using from command line multiple vault passwords.
Let me know please if this helped or maybe you have further queries.