why ansible-playbook ask for a vaut password, whereas target hosts does not need ?

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Hi everyone, 

I'm new to Ansible, I'm using 1.9.2 on Centos 6.6.

Here is my playbook :

---

- hosts: server1

   

  tasks:

 

  - name: retrieve files from remote server

    synchronize: src=/etc/myfile dest=/tmp/res/ mode=pull

Here is my  /etc/ansible/hosts file :

[grp1]

server1 ansible_ssh_host=10.0.0.1 ansible_ssh_user=admin

[grp2]

server2 ansible_ssh_host=10.0.0.2

server3 ansible_ssh_host=10.0.0.3

server4 ansible_ssh_host=10.0.0.4

And my host vars, encrypted with ansible-vault, containing sudo passwords (whatever...) :

/bin/ls /etc/ansible/host_vars/

server2

server3

server4

As you see, "server1" does not have configured vars, because it does not need.

Here is what I get when invoking ansible-playbook command :

user@ansible$ ansible-playbook playbook.yml

ERROR: A vault password must be specified to decrypt /etc/ansible/host_vars/server2

But "server2 is not in the targets of the playbook.yml !!

What did I miss ?

Thanks for answering !