[amul]

I've had the same issues with the login system as many people here.
When I went to the 4images page it would recognize the users logged into the forum and display them on the bottom of the page, but I would not be logged into 4images itself.
Because of that it also didn't display the admin control panel link for me,
and when I went to the admin index.php file manually, it asked me to login, after which I got reverted to the forum home page and was still not logged into 4images.

I discovered that I had changed the cookie prefix when installing VB, so after I edited the configuration part in sessions.php to the correct Vbulletin cookie prefix as it is listed in your config.php file for Vbulletin, it worked great.
No more login issues and control panel link shows up.
I have the session timeout set to 1800, but I really don't think it matters much as long as both the vbulletin and 4images timeout are the same.
Hope this helps anyone.