This is just my opinion from watching the scene, but I would think the shot where she goes to hit Rhett and then falls is a stunt person because that is clearly quite a longer tumble and we can’t see her face as she falls forward and down. But I think the last shot where she lands and it is clearly Vivien Leigh’s face was really her; it looks like she only had to throw herself down a few steps so she probably managed that, plus her costume could have hidden some padding and whatnot.