If you are having problems signing in to your Vanilla 1.1.2 installation, this discussion should help you.
As of Vanilla 1.1.2, Vanilla applies the value stored in the COOKIE_DOMAIN setting to php sessions. If you don't have this value set properly, you will not be able to create PHP sessions, and you will not be able to sign into Vanilla.
The most common problem is that people set up their installation of Vanilla on an intranet with an alias like localhost or some other name relevant to them like intranet or in my personal case at home markisawesome.
The reason this type of a COOKIE_DOMAIN setting won't work is because cookies require what I'm going to call a "dot-dot" domain in order to be saved properly - domains like www.lussumo.com or .lussumo.com.
You may not be in a position to use a dot-dot domain, and that's okay. You can go into your conf/settings.php file and change your COOKIE_DOMAIN setting to this:
$Configuration['COOKIE_DOMAIN'] = '';
Then the cookies will be saved properly and your sessions will be saved properly too.