It would be up to interpretation, but he's just saying it would be hard to recognize facial expressions with his helmet on. The whole expressionless face 3PO thing worked well in the movies because 3PO could portray his emotions through expressive body language. But Vader wouldn't be able to show surprise without either his voice or his body movements, and I just can't see Darth Vader raising his arms up in the air and leaning back like C-3PO would. Well... actually, I can, and it's pretty damned funny!