My guess would be since it is browser specific to IE9, that there is probably some browser add-on or control that needs turned on for it to work. Definitely sounds more like a client-side issue to me, since Firefox doesn't even have the icon.
Ie9 does not have a built in spell check. Ie10 does. IE9 requires you to download some type of IeSpell program. For whatever reason Vbulletin shows that to IE users so they can also have spell check. Never knew it was there as I use firefox which has a spell check built in. . From what "m reading at vbulletin the feature was supposed to be removed, the functionality was but the button wasn't. They recommended people use a program like IEspell here. http://www.speckie.com..