vwebp: 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/vwebp - from package: libwebp-tools

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

On Debian possible default full paths are:

/usr/bin/vwebp - from package: webp

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

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