Why could the encrypted Ansible files not be decrypted although the vault password file has been set?
I found an issue with Ansible/Python causing a failure to decrypt.

Specifically, Ansible 2.7.10 and 2.7.13 under Python 3.6.8 on Ubuntu 18.04 would not decrypt (using ansible-vault edit -vvvvv) a vault file that was identical to file that decrypted OK on macOS 10.13 using Python 3.7.2 with same Ansible versions.

Upgrading to Python 3.7.3 on Ubuntu (using apt-get install python3.7 on 18.04) fixed this.

Your issue may be different, but if on Python 3.6 or lower, I would try upgrading to 3.7.