vnstati: 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/vnstati - from package: vnstat

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

On Debian possible default full paths are:

/usr/bin/vnstati - from package: vnstati

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

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