Does anyone have experience of linking Wordpress and vBulletin?

We're looking at linking the login details so people can login to the vBulletin forum directly from the Wordpress 'site' which will be acting as the home page and news stories etc CMS.
My colleague attempted to install and activate a plugin and it said it was missing the mcrypt - but I have NO idea what this is..? Any help appreciated massively! Thanks in advance.