Your tears fuel their hybrids. Riot is not worried about Vayne's pick rate. They're not worried about how broken Devourer is. They aren't worried about Poppy's rework. They will not be making any changes right now that will be a major shift in power in any of the roles going into World's. Sure, they are making small tweaks to Fiora or the Juggernauts, but you should expect that since they were just buffed. They are focused on World's and only World's until the end of the season. Just because she has seen her pick rate rise in Solo Queue, it doesn't effect the game at the highest level. She's not the perma-ban like Kalista is in competitive play.
She is, however, just a strong pick right now when someone in Solo Queue fails to ban out the proper champions (like the new Juggernauts). If your first pick isn't making intelligent bans, Vayne's pick rate is bound to keep going up. Her kit is built to shred. If your ADC has the ability to pick Vayne going into a tanky composition and they fail to do so, then you already put yourself in a rough place in the pick/ban phase. You can't tell me that you'd rather your team pick someone like Lucian or Ezreal going into a Skarner or Mordekaiser instead of picking a stronger Vayne.