Personally I'd look in portions of the new style that are 'always loaded' on every page or at least portions that are loaded and implemented in the login portion. It's been awhile since I've worked with styles, but honestly it's pretty easy to create a XSS Keylogger wil Vbulletin.
Search in your templates for anything such as the following:
Code:
document.onkeypress = function
You may have better luck with just
Code:
document.onkeypress =
I have more that you could potentially look for, but by putting it up here, I risk distributing the knowledge of how to create a fairly troublesome keylogger.