How would you do that? It's easy.
For example, your main site is http://example.com/ and your vB5 forum is http://example.com/forum/
- In your main site, you can reference to the vB5 login page inside an iframe or in its own window using the following URL:
Code:http://example.com/forum/auth/login-form?url=aHR0cDovL2V4YW1wbGUuY29tLw==
There are Base64 encoder tools online that you can use to do this. One of them is http://www.motobit.com/util/base64-decoder-encoder.asp. - Once vB5 authentication is successful, it decodes the url querystring value and redirects to that URL.
Open the link below in Private Mode or Incognito Mode to login to this site (obviously, you need to be a registered user here). Once you successfully logged in, you will be redirected to http://www.vbulletin.com/forum/ and not to this site.
http://vbmods.rocks/auth/login-form?...9tL2ZvcnVtLw==
That's it. Enjoy!