When Rey spoke, it was Vader's turn to be confused. He doubted Obi-Wan could hear her - was certain of it - and didn't see the point of calling him Ben here.

He was still confused then his grandson responded even if he caught up quickly - right, birth name, so on. Leia, that..
he probably DESERVES that but still.

Vader withdrew his hands, nodding at them both. "Grass. Or water. Growing, moving things." He knew where was easier to grab hold of the Force and ground one's self. At least in the Light, for her.