It is a real head scratcher that one.
Short of replacing Vader with a suitably suited actor arriving suffering a similar but less staged fate and seamlessly blocking Mark's moves, I can't think of a solution either.
It's not as if much of that part of the fight would need replacing (it's just getting him into that position and removing the obviously fake hand on a stick as in Angel's mock up).
He his on his back almost as if in a slide, which would suggest more of a kick or a Force Push than a natural reaction to being bashed as seen in the original version.
It does show how much more stylish and thought out the saber duel in ESB was.