Darth Vader stood on the command bridge of the Mustafar control center, hand of
durasteel clasping hand of flesh behind him, and gazed up through the transparisteel view wall at the galaxy he would one day
He paid no attention to the litter of corpses around his feet.
What does Vader do when Luke is affectionate towards him?
well, physically, he goes very, very still. like a deer caught in headlights; luke is a fearsome tyrannosaurus who won’t see him if he doesn’t move. he’s trying to meet his life’s dream of melting away into nothingness. YOU CAN’T SEE HIM LUKE HE’S A FLOWER (of darkness)
mentally, it’s more complicated. if luke says, “i love you,” vader’s immediate reaction is, “no, you don’t.” because luke doesn’t know the extent of vader’s crimes, luke doesn’t know what he’s saying, vader doesn’t deserve it, luke’s not old enough to understand, luke’s lying - all these things would run through his head. luke can’t love him, because vader hasn’t earned it - anakin never learned that you don’t earn love. (ah, yes, the slave background strikes again.)
but, of course, vader is just as thirsty for affection as anakin ever was. he treasures it. vader’s at that strange juxtaposition where what he wants and needs - love - is something he doesn’t deserve (which may or may not be true, depending on your personal views - but vader himself doesn’t believe he deserves it), so while he doesn’t quite believe in it, he’s touched by it. one of the most evocative images i’ve ever had is someone describing vader holding a holo of luke to his chest, and they described it as a pale candlelight over his heart - the first resurgence of the light in vader.