That's the other part of your reasoning I don't understand. All through out the original trilogy Vader is a very angry dude who often lashes out in violence. That does not sound like an emotionless machine.
The only thing I can figure is you're basing this all on the line, "He's more machine now than man." But that's not the entirety of the line, the whole this is: "He's more machine now than man, twisted and evil." Again not something that describes an emotionless robot.