Should work fine after you change the base URL of your vBulletin forum to use HTTPS instead of HTTP.
At the vBulletin settings > Server Settings and Optimization options you should also configure a custom Remote YUI Path such as
https://ajax.googleapis.com/ajax/libs/yui/ which uses HTTPS.
Also you may be receiving mixed content errors when there's embedded media such as images/videos on a post which loads stuff through HTTP.