Gareth Edwards spoke to Yahoo Movies:
"What was added — and it was a fantastic add — was the Vader action scene, with him boarding the ship and dispatching all those rebel soldiers," he told Yahoo Movies. "That was something conceptualised a little later."
"It was a really great punch in the arm and something I think fans wanted to see," said Gilroy.
The original cut apparently was just going to end with Jyn (Felicity Jones) and Cassian (Diego Luna) passing on the physical copy of the Death Star plans, which would eventually end up in a digitized Leia's hands.
I do love how they added the Vader sequence to spice things up the final moments of the film. As I've previously mentioned in this site, the last 20 or so minutes of "Rogue One" are the best moments of the entire film. They link up to "A New Hope" in such a masterful way and the Vader sequence made me feel so giddy. I mean, who doesn't want to see Vader using his skills to kick ass .... No matter how evil the son of a bitch is.