Its more that with Vayne, people think its hard to win games with her until you hit plat
You do % health TRUE DAMAGE, all your hits will do over 500 damage against someone with 300+ armor
You have a stealth+reposition every 3 seconds or so, meaning if you time anything at all correctly you'll get the kill on anyone who dives on you
You have a mistake button called condemn and that will save you pretty easily if all else fails
Also you can chase anyone forever. Yea, not a very hard champion to play, especially with the new masteries.
Riven is quite bad in this meta, like you really can't play her effectively at all after higher plat elo where everyone has good enough reactions and is good enough with cc that its an accident if you get anywhere close enough to do damage in fights. You pretty much have to build 2 or more defensive items every game to have the same impact something like renekton does.
Vayne mains are more common than ever, and Riven mains are starting to disappear because shes quite weak right now.