Have him say, “Padme once thought as you do”.
You can use “Padme” from ROTS when he asks the Emperor about her. It’s loud and deep, and Luke should know his mother’s first name.
The issue is that the audience doesn’t know her fist name (depending on the viewing order at least) and it locks line into only making sense if you watch the prequels, which some people don’t like to do. More over the audience doesn’t know Luke knows, she is a mystery in to him. He ask Leia about her because he knows nothing and Luke’s reaction isn’t great enough for someone who has just found out his mother’s name. In context it would also make more sense for Vader to say “your mother” as it is father to son talk.