I do know that it has something to do with the lvl of affection you have with each of your companion characters. However I do know that there are certain companion characters that have a deeper background than most of the others you have beacuse they play a key part of your characters main story. All anyone can do is suggest that you try another Sith Warrior and work on your other companions other than Vette.
I've gotten 3 other companions to 4000 affection. All they want to do is talk.