Xue Yang briefly remembers the vine and glances up quickly, but it doesn't look like it's tight enough that he needs to worry about it killing a cultivator at this point. So he lets it fall back out of his mind again as he continues thrusting and giving more gasps. Some are honest, some are added more for effect.

Either way he focuses on the feeling of Song Lan's cock in his hand. He allows himself to fantasize about all the things he could do to it with Song Lan restrained like this, but once he's done indulging, he files them all away for the moment. That's not his goal right now. Instead he focuses on scaling up his own masturbation habits to someone of Song Lan's size and starts deliberately stroking, picking a pace and technique that feels good for himself.