I highly doubt you fully grasp all of Vayne's mechanics if you just picked her up and found her "easy to play." You may know the basics, but I doubt you're even close to mastering her. Or, who knows, maybe you're one of the rare people that can play Vayne well from the get go.
Ahri and Lux... well, their mechanics are usually referred to as mid level to easy for a reason. Most of their spells are stupidly easy to land (I mean, do you see the range on that slow??)
Riven is hard for me to say anything about because my playstyle doesn't go well with most melee champs. From what I can gather from her mechanics though, she does have some unique things that probably should have been thought through better before launch (like, I don't know, making her manaless, giving her a free tripple jump, and giving her a shield that SCALES WITH AD for a start).
Also, sometimes when people play "op" champions they do sometimes do better against them because they usually go up against something that has a chance to really ruin their day, and from there they can use that strategy to get better at facing off against those "op" champions.
Every champ has a weakness; you just have to find it ;)