"You look better." He isn't aware of Vader having been doing more than was likely wise during his absence, though Kylo wouldn't have faulted him for it. He'd do the same thing under the circumstances.

That Rey's healing seems to have indeed helped is a relief on a few levels. He'd be glad for it, no matter what, but perhaps especially glad now that he has a lot of thoughts on his mind to perhaps talk out, and Vader seems perhaps in a better state for that conversation.