I'm rather surprised, since I never saw any spell-checking on vBulletin. Searching reveals the two most recent threads dealing with this on the vBulletin.com site:
Link 1,
Link 2, indicate it has not been a feature for a long time, except that for users of Internet Explorer there was a icon
ABC for locally downloading IESpell to their browser. Or it might be the alternative, iespell: one was OE's own checker extension, and the other was a third-party addition; and...
they both had the same name. Well done, Microsoft !
Firefox's own spell-checker appears to work automatically on vBulletin.
It's a sad and terrifying thought that each minute someone, somewhere, is using Internet Explorer for the first time and getting hooked into a vicious addiction that will drain their web experience and leave them mindless drones controlled by Redmond.