What do you think Vader would do if he caught Luke crying alone about something? Maybe about all that he has lost, or something about padme...
Be very awkward and probably try to sneak away, honestly. He’s extraordinarily out of practice when it comes to emotions and comforting people. I don’t know that he was ever very good at it. He doesn’t know healthy coping mechanisms for himself, so he’s not gonna know how to handle someone else crying either.
If Luke’s been with Vader for a while, he might awkwardly pat him on the shoulder or something though. Or ask if there’s someone he needs to kill.