The vacant lot makes several appearances in the book, and a few in the movie. Johnny and Ponyboy fall asleep there once, and the rumble is also held there. It seems to be like a hide out or safe place for the boys whwen they have no where else to go.
When Dallas Winston, or Dally, is on the run from the cops at the very end of the novel, he calls Darry and asks for him and the boys to meet him at the lot so that they can hide him. Sadly, by the time Darry and the rest of the gang get there, Dally has been shot down by the cops.