Xvnc: 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/Xvnc - from package: tigervnc-server

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

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