You can still get it, you probably need to get further along Diakor's route till you become his oracle. You can't get the scene where you were already his oracle before talking to Garthoz however. If you got the Diakor wanted to see you and you don't get told to relieve Garthoz you didn't do enough for Diakor.
You're probably missing something with Vandeel or Vince, as aside from doing all of Vaorn's content, the only thing needed is progressing in their content as well.