Oath upon oath
The Mind's Eye card had failed when Nero tried to use it in the fog, so chances were high that he might not be able to use it to peer past the vault either, but there was no harm in trying. More importantly, if he was able to get a sense of what the vault was hiding, his motivation to execute this plan successfully would grow immensely.
Nero walked up to the vault door but suddenly paused when he felt the aether in his body tremble, resonating with a certain hum in the door. He had never felt something similar before, and for a moment felt like his body was reacting to the door. But it wasn't. It took him only a moment to realise that his body was reacting to whatever was behind the door, and the gentle hum was coming from behind it as well.