Vivec was the only one of the Tribunal who married and slept with Molag Bal to learn how to do it. He was probably the only one of the Tribunal who was narcissistic enough to not zero sum. It takes a huge will, and an even bigger ego to achieve CHIM. Dagoth Ur had a sort of reverse CHIM. He used the tools of Kagrenac wrong, and ended up with his waking body in the Dreamsleeve, and his sleeping body in Mundus. The Dreamsleeve exists in Aetherius, and when you sleep, that's where your soul goes. While in the Dreamsleeve, your soul has essentially infinite power. The Dreamsleeve is also where dead souls go to be recycled and reincarnated. So, when Dagoth Ur is awake, he's now in the Dreamsleeve and powerless, and when sleeping, he's now in Mundus and all powerful. That's why the Tribunal couldn't do anything to him. While in Mundus he was lucid dreaming, and had full control of the dream. Even if they killed him, he'd just wake up from his dream.