vpnuser: 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/vpnuser - from package: pptpd

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

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