While having R apply Denting Blows was thematic and felt great as part of Vi's combo it was just waaaaay too powerful. Assuming the guy you ulted didn't have his team around to protect him R,AA,E,Q,AA,E was an almost guaranteed Double-Proc on W (with the last Q,AA,E all benefiting from the armor shred). Too much burst.
Sure, could have kept the stack on R. Combo would have felt great. In exchange would probably have had to nerf damage on W, E, and/or Q tremendously. Would have ended up making Vi more binary. Is my ult up? I burst you for all your life. Is it down? Looks like I have nothing to do since I need R to effectively combo.