Well if the post you made immediately before that one hadn't been so obtuse (but I'll get to that); I might've extended a bit more help than the blunt line above.
Anyway, it seems Dave might have a solution for you; and as I'm in a position to ignore other things a little better today, I query whether the internal IE spell check struggles with a) the quick reply box only, b) the advanced reply box only, or c) both. I could imagine it might struggle with the quick reply box, but work OK with the advanced reply box, given that the former is spawned via AJAX and the latter more standard means (IE really does struggle with anything remotely innovative).
I'd wager the reason you don't have issues anywhere else is because they're likely not running vBulletin 4 - which like it or loathe it, it (or technologies built-around it/based-on it) will spread sooner or later.
I can say it's extremely unlikely that we'll ever go backwards through the software development line; it might transpire that the makers of vBulletin capitulate and put a boat load of extra code into it in order to wipe IE's dirty little ***; but to be absolutely honest, I really hope they don't - the more sites there are which simply conform to standards, the greater the motivation for Microsoft to sort out their sh*t. Perhaps that might mean that the function in IE you're bothered about, is re-coded to work in a broader range of circumstances. In the meantime, I'm afraid you'll just have to use one workaround or another...
Regards,
Rob.