ansible-vault: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/ansible-vault - from package: ansible

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ansible-vault - from package: ansible

You may install the required package with command apt-get install {package}

Error in other languages:
ansible-vault: bevel nie gevind nie
ansible-vault: comandă negăsită
ansible-vault: comando não encontrado
ansible-vault: commande introuvable
ansible-vault: command not found
ansible-vault: käsku ei ole
ansible-vault: không tìm thấy lệnh
ansible-vault: komanda nerasta
ansible-vault: Kommando nicht gefunden.
ansible-vault: kommandot finns inte
ansible-vault: komut yok
ansible-vault: nie znaleziono polecenia
ansible-vault: níor aimsíodh an t-ordú
ansible-vault: no se encontró la orden
ansible-vault: no s'ha trobat l'ordre
ansible-vault: opdracht niet gevonden
ansible-vault: parancs nem található
ansible-vault: perintah tidak ditemukan
ansible-vault: príkaz nenájdený
ansible-vault: příkaz nenalezen
ansible-vault: команда не найдена
ansible-vault: командата не е открита
ansible-vault: 命令找不到
ansible-vault: コマンドが見つかりません