That part never made any sense to me in the movie. I mean they had both been all over the place during that fight, both of them had the high ground multiple times. And then after that wild fight, suddenly the high ground was the major difference.
They had been doing acrobatic moves and blocked saberblows from all kinds of angles and positions, during flip-flops and what have you. Then suddenly that jump was an "impossible" move to succed for Vader. Made no sense to me.