1 Darth Vader: Luke... help me take this mask off.
2 Luke: But you’ll die.
3 Darth Vader: Wait, what? Really? Nobody told me that!
4 Darth Vader: Leave it on! Leave it on! Call me an ambulance! Medic!!

I remember watching this scene in the cinema in 1983 and wondering how Luke knew that Vader would die if the mask was removed. It wasn't obvious to me as a viewer that this should be the case. Indeed in The Empire Strikes Back we'd seen Vader briefly without the helmet on. It was logical to assume he could remove it safely. How did he eat or drink otherwise?

So there are two unanswered questions here. Why does Vader die without the mask? And how does Luke know that?

The most sensible answer is that Luke was making it up to scare Vader.