Vader hadn't been looking directly at Luke as they talked. Mostly he was looking out the viewscreen of the transport, but honestly he wasn't really watching out of it. Paying enough attention they wouldn't hit anything or have an issue, but

Even while conversing with Luke, his mind is elsewhere.

Enough so that Luke's curse draws him back, so that he looks over at Luke directly, chin tilting down just a little to make eye contact. And the light from Naboo catches his lenses enough for Luke see inside, the ruined scared skin around Vader's eyes, his irises appearing almost purple as the natural blue combined with with red.

"I did die. And revived to have nothing." Until now.