Sorry if anybody already posted this answer
But what happened was Vader didn't blow with on the death star explosion. Luke pulled him out and later cremated him. So its not a big surprise that helmet made of some extraterrestrial metal wouldn't be left in ashes, Luke probably kept his father helmet and is referring to him in past tense, because theirs little probability he lived. He was hit with gamma from the sideous explosion death while sustaining heavy force lighting, then taking his helmet off and off life support, then being cremated. Though he might still be alive it is a possibility.