After doing the questline for Bel-Betu, and not killing Vildaryn Terano, just telling him to leave progressed the quest since I was above level 15 when I threatened him to leave. I completed Blood Ritual of Volenfaryon, and afterwards killing Bel-Betu. I then brought an extra labyrinth key to Vildaryn and he entered and the voice lines/script for Vildaryn to die still went off. I was hoping for some secret dialogue regarding him being dead/a crystal or something. I checked on UESP if I maybe broke something, and saw nothing regarding this happening.