Sounds like you are trying to utilize the Guest User which is available for "core" license servers. It needs to be turned on by a Server Administrator as well. Take a look at the "share" link for each Viz to see the formatting of the embedded URL parameters. That being said, if Guest is turned on and a user attempts to go to a non-embedded view, the user will typically be redirected to the guest page. If you are embedding the page and being prompted for authentication, the Guest account is not being used or has not been added to permissions on that view.
Hope this helps!