Unfortunately it appears that when Ansible hands off the db_password integer value 123 to the Trellis to_env filter’s dict_value, it comes out as the python value None. Perhaps Trellis could detect such a scenario and output a help message:
Numeric values in env vars in wordpress_sites and vault_wordpress_sites must be wrapped in quotes: db_password: '123'
Try making that change then run the last portion of your dev.yml playbook: ANSIBLE_TAGS=wordpress vagrant provision