Vayne is and has always been regarded as a prime late-game duelist and tank buster, but looking at all of my recent games, actual tanks have been extremely scarce. When considering her pick and win rates, it should be in her core design that neither should be high in a meta where there are simply no tanks. As much as people complain about Riven, myself included, she (Riven) is the exact opposite in that she should be extremely strong in a meta without tanks.
My main point here is that I believe that seeing both popular should be a very strong indicator that something is very wrong. Players should never feel like they can't pick a tank into Riven because Vayne is strong, but also feel bad about picking Caitlyn into Vayne because Riven is strong. The boards have been in a bit of an uproar about damage being too high and tanks being weak and frankly, I think this is one of the clearest examples validating those concerns.
I am not saying Vayne didn't need some love, but she should not have been given the Q cooldown buff during her ultimate without consideration of how buffing her into viability outside of a tank meta would push tanks completely out, thus allowing champions like Riven to thrive to the extent that she does. I for one, would really appreciate if someone at RIOT could address the situation and explain the design philosophy behind it, because I'm really not sure what's up. Kayle, with her true damage, should also be fairly weak in a non-tank meta, but she is doing very well too.
All in all I just sort of feel in the dark as to what RIOT's design values are right now because all I have seen in the past few patches is buffing damage at the expense of defense due to pro play specific balancing and I think it's really hurting everyone else. I'm not trying to complain about champions being busted, just something really does feel off with those two champions being so common.