vnstatd: command not found

Have You tried accessing the binary with the full path?

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/vnstatd - from package: vnstat

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

On Debian possible default full paths are:

/usr/sbin/vnstatd - from package: vnstat

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