http://docs.ansible.com/playbooks_variables.html#conditional-imports

inv

localhost

result

$ ansible-playbook site.yml -i inv

PLAY [all] ********************************************************************

GATHERING FACTS ***************************************************************
ok: [localhost]
localhost: importing /Users/.../vars_Darwin.yml

TASK: [show vars] *************************************************************
ok: [localhost] => {
    "msg": "test_var=b"
}

PLAY RECAP ********************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0

site.yml

- hosts: all
  vars_files:
    - [ 'vars_{{ansible_os_family}}.yml', 'vars_b.yml' ]
  tasks:
  - name: show var
    action: debug msg="test_var={{test_var}}"

vars_b.yml

---
test_var: 'b'

vars_Darwin.yml

---
test_var: 'Darwin'