What you did to the Black Heart was only a battle victory that dealt a serious blow to the Vex. They are far from defeated though and a new Black Heart will probably regenerate somewhere else. Destiny's ending is just the beginning of this saga. Like the Exo stranger told you at the end, the real fight is out there. The dailys, patrols and strike missions is the ongoing war against the Vex, Cabal, Fallen and Hive. The Vanguards won an important battle but the war is far from over.