Windows 10 has its own VPN client. If you forget the saved VPN password you cannot retrieve it.
So, to retrieve the saved password you have to take help of a third party application known as Dialupass.This utility enumerates all dialup/VPN entries on your computers, and displays their logon details: User Name, Password, and Domain.
But there is one negative point-Some Antivirus programs detect Dialupass utility as infected with Trojan/Virus.Click here to read more about false alerts in Antivirus programs