Heh, we don't want you to log out
If you go to the home page of the forum, it's on the bottom right-hand side. There's a row of links there: "Mark Forums Read" "View Forum Leaders" and "Log Out"
If you're not on the home page, you won't be able to see a log out button.
We're upgrading to the latest version of vbulletin next week. When we do that, we'll be making some minor alterations to the theme, including adding another log out link.