nice idea, i'd be interested if this was created, coz it's nice to have the minimum post count so people can't just post a letter, but a picture can say 1000 words...
maybe something can be created so that vBulletin counts an attachment as 'x' letters, so if we define an attachment to equal 10 letters, then it will think you've typed 10 characters, and bypass the 'minimum post count' rule