In the past I've been a big phpBB fan, but recently I've wanted a leaner meaner discussion forum for integration into an existing site/application. I stumbled upon Vanilla.
So far, I'm a big fan. I did have some trouble getting setup though. When installing, you set the COOKIE_DOMAIN, which php then uses to store your cookie. This needs to match the url through which you are accessing the site. If it does not, the authentication will work but you will not be signed in. In other words, if you type in the wrong password you will be rejected. If you type in the correct password you will be forwarded to the main page, but you will still see the option to sign in up in the top right hand corner.
Specifically this will happen if you have it deployed locally and access it using "localhost". In this case, simply map your deployment domain to your local IP address in your /etc/hosts file.
Also make sure your conf/settings.php contains the correct domain. I searched for localhost in that file and made sure all were changed to be the deployment domain.
I hope this helps anyone trying to get started with Vanilla.