Most times I see a Riven go mid, it's an explicit counterpick.
e.g. Riven absolutely facerolls Kassadin. Riven does have a few other particularly strong match-ups mid as well (mainly the melee mid laners, as Riven can kill them in lane pretty easily at levels 2-5).
Same goes for the other Top lane bullies that go mid. They're counter-picking specific mid laners.
Vayne does well top right now because she's just strong in general, has good mobility and kiting, and is a solid counter-pick to the seemingly omnipresent juggernauts top lane. I'd imagine Quinn does the same but just isn't as naturally strong.
I hate the Yasuo top pick in general, and have never seen Ekko bot (although I'm not a fan of Ekko Jungle either). For the most part, those two just have much better options who do effectively the same thing in that lane. Yasuo works mid because he makes good use of his Wind Wall against most mid laners - it isn't very effective top. There are other assassins who are stronger junglers than Ekko (and definitely much better damage-oriented supports).