vlock-main: 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 Debian possible default full paths are:

/usr/sbin/vlock-main - from package: vlock

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

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