vpnc-helper: 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/sbin/vpnc-helper - from package: vpnc-consoleuser

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

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