vkill: 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/vkill - from package: util-vserver

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

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