The vast majority of websites offering free authentication codes distribute trojans, ransomware, and spyware disguised as key generators.
: This is your best bet. As explained in the official documentation and by multiple users on the Creative COW forums, the proper and most reliable method is to go to the official Sony site and log into your account. The serial number assigned to you upon purchase should be listed there. If you purchased a boxed version, you would have received a physical copy of the code inside the packaging. Sony Vegas Pro 8.0 Authentication Code