obi went down for 2 reasons, 1 he was old and not in his best abilities to fight, you could tell by his lightsaber the force of his power had diminished. 2, he knew that dying an honorable death and showing luke the true jedi ways would provoke his spirit and give him power he had yet to see. as for vader, he didnt disappear true, perhaps because he was dead long ago, mostly machine anyway. but in the end he still joins the good spirits of the force, obi, yoda, and anakin all reunited.
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