I've never been able to think of Darth Vader without his helmet. To me, that skull-like breath mask is his face. When Luke removed Vader's helmet in "Return of the Jedi," he ceased being Lord Vader and became Anakin Skywalker again. I've heard the same said about Boba Fett. Exactly what he would look like without the helmet doesn't matter because that helmet is his face. Without it, he's not complete, not Boba Fett. I think the same can be said for Vader.