---
- hosts: testgroup
  gather_facts: no
  vars_files:
    - "vars/common.yml"
    -
      - "vars/{{ foo }}.yml"
      - "vars/defaults.yml"
  tasks:
    - import_tasks: validate.yml

- hosts: testgroup
  gather_facts: no
  vars:
    _vars_files:
        - 'vars/{{ foo }}.yml'
        - 'vars/defaults.yml'
  vars_files:
    - "vars/common.yml"
    - "{{ lookup('first_found', _vars_files) }}"
  tasks:
    - import_tasks: validate.yml