vgextend: 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:

/sbin/vgextend - from package: lvm2

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

On Debian possible default full paths are:

/sbin/vgextend - from package: lvm2

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

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