statement in chapter twelve because he wants Ponyboy to go back to the way he was. He sees a lot of potential in Ponyboy and does not want to see it wasted. Here is the quote:

You're not going to drop out. Listen, with your brains and grades you could get a scholarship, and we could put you through college. But schoolwork's not the point. You're living in a vacuum, Pony, and you're going to have to cut it out. Johnny and Dallas were our buddies, too, but you don't just stop living because you lose someone. I thought you knew that by now. You don't quit! And anytime you don't like the way I'm running things you can get out.

This is Darry's version of a pep talk. He is clearly concerned about Ponyboy. Ponyboy has had thoughts of quitting school because schoolwork is understandably difficult for him now. Darry is telling him that he cannot just give up on life and his dreams just because Dally and Johnny are gone.

Following the tragic deaths of Johnny and Dally, Ponyboy becomes emotionally callous and stops trying to succeed in life. Ponyboy has difficulty focusing and his grades dramatically plummet. Darry eventually addresses Pony's lack of concern and effort by telling him that he is "living in a vacuum" and encourages him to start trying to succeed in life. Darry's comment regarding Ponyboy living in a vacuum essentially describes Pony's narrow perspective, mindset, and emotional status. Pony living in a vacuum essentially means that he is simply going through the motions and acting like he is living in a bubble. Pony's callous demeanor and the "vacuum" surrounding him are psychological defense mechanisms that protect his fragile emotions. Darry's comment draws attention to Pony's lack of focus and concern for life. Fortunately, Pony reads Johnny's letter and takes a new, positive approach to life that helps heal his emotional wounds.

Despite growing up without parents, Ponyboy was actually quite sheltered. His older brother, Darry, protected him from many of the harsh realities of life. Because of this, Ponyboy was given opportunity to be the dreamer he was. Ponyboy did not recognize many of the outside factors that caused his situation. For example, Darry could have gone to college on a scholarship but instead chose to stay home, working a job he hated, to raise his younger brothers. So Darry was making sacrifices for Ponyboy, but all Ponyboy was only able to see was that he didn't live up to Darry's expectations. It was stated that Darry was always a Soc, but Ponyboy did not realize the true implication of this. Darry had a different mindset than the Greasers. Darry recognized that Ponyboy had a chance to escape the life of poverty and possible crime that so many other Greasers endured. This is why Darry thought Ponyboy seemed to be "living in a vacuum." 

What Darry means by this is that Pony is ignoring the realities of his life.  He is acting as if there is nothing going on in his life, like it is a vacuum that is empty except for the deaths of Johnny and Dally.

Darry is telling Pony that he has to quit acting like this -- he has to realize that there are all kinds of other things going on that are important.  He has to realize, especially, that school is important and that he could make something of himself if he really starts to apply himself to school.