After a thorough search, we have found there is no built-in spell check for VBulletin. There are some "hacks" that can be done to "add" one, but we do not want to risk damaging or disabling Pet Talk that way.
For now, people can do spell check the old-fashioned way. Keep your favorite word processing program open, type your post, copy into the word procesor, spell-check it, then post it! Takes a little longer, but well worth it.