Sora actually gets a little worried expression on his face when Vanitas says 'when they're cut down I know it instantly. He frowns and rubs his chin in thought.

"You know it?" he asks. "Does... does that mean you feel it in some way? Does it hurt you?"

Now he's going to have to have a crisis about hurting Vanitas over and over again without knowing it, isn't he? Sure, he has no regrets about fighting Vanitas head on, but hurting him at a distance feels wrong somehow.