// encryption for group_vars/servers.yml by ansible-vault
$ vi group_vars/servers.yml
...
...
$ cat group_vars/servers.yml

$ echo <YOUR ANSIBLE-VAULT PASSPHRASE> > ./.ansible_vault_pass
$ ansible-vault encrypt group_vars/servers.yml --vault-pass ./.ansible_vault_pass
Encryption successful
$ cat group_vars/servers.yml
$ANSIBLE_VAULT;1.1;AES256
32653934343964336466393561396535323863303134663533653063333530366132663762623638
3730376538353365643762373639386365646631303132360a333666626337613837376639313233
38616562613638393739353966313035386266313033343764613733333461313339626566666464
3365613035636239350a623532353665666231633630346232623130633630643564623265353033
61353131636231633032363435343665383037336636353034303931313630626336333030383961
61376633663633316335373765373337333731633431313661383730393135333631366632386139
39613661343138623939336235316665633431616163393261636338323163633861623565373033
38656665336164313337316666613334303534363931666365613061313062643337396434336334
37336131306364656333663964343763653737396438613539353563346138653363

// decrypt
$ ansible-vault decrypt group_vars/servers.yml --vault-pass ./.ansible_vault_pass
Decryption successful
bash-3.2$ cat group_vars/servers.yml
---
ansible_connection=ssh
ansible_ssh_user=XXXXXXXXXXXXX
ansible_ssh_pass=XXXXXXXXXXXXX
ansible_sudo_pass=XXXXXXXXXXXXX