vlock: 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/vlock - from package: vlock

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

On Debian possible default full paths are:

/usr/bin/vlock - from package: vlock

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

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