When Vex turned 15 her mother died of an unknown disease now known as the Vulcrux. They believe it occurred after the impregnation. She was devastated and with no role model she has to find another. She was begging for food, and homeless. She didn't know where to go but the void. She started stealing runes and magic equipment in the void. She would steal in the overworld and just go through the portal and go to the void. She was bouncing from dimension to dimension just to live. She finds it exciting yet rewarding. I guess that's the life of a voidwalker.