Disney puts movies in the "vault" for a couple of reasons. One being the obvious, just knowing that you have a limited time to purchase the movie gives you a heightened sense of urgancy about having to purchase it before it goes away.
Another reason is that by taking a movie away from the store shelves for years at a time (usually 10 year cycles) Disney insures that the new generations will become excited about the movies all over again.
The Lion King should still be available at most stores until January 31st 2005. After that it is going in the vault, but most stores will still keep them on their shelf after that time if they still have a few copies (price may go up though).