You will just have to wander around on the world map, until you get a special encounter called "The Guardian of Forever"
You will find a portal. Enter it.
You are in Vault 13, the third floor. Go down the hallway, and turn right (on the screen). Open the door and go up. Open the other door and the Solar Scorcher should be in a locker, on the floor or something. Pick it up. Then head out of the room and go right. Use the computer that has lights on to exit Vault 13.