(Why can't I VIP a new thread?)
The VIP checkbox allows you to respond to a thread without bumping it to the top of the list. It's considered polite and makes you a VIP. Your post is decorated with a star and you get all the honor a VIP deserves. It actually does the same thing as entering the word "sage" in the mail field of 2channel or its offsprings (you can read it as "do not bump"). The first post of a thread cannot be saged, because new threads always appear to the top of the list.