Hard champions vayne, rek'sai, zac? Like for real? There is no way to help you here. Riven is pure micro mechanics, easy to play but a Little bit harder to master so it's only about your practice. The same with zed, he is extremly easy on lane, easy farm, good range on free spells, but in late game he requires micro mechanics and kind of prediction to kill an enemy in the middle of enemy team during teamfight so it's only about your practice. Rek'sai and zac are very simple, the only thing you have to know is which camps and how to do in which minute, you can watch some pros doing so. Rest of their kit is the same as any other tanky jungler and you are doing exactly the same thing.
If you want to be good with vayne you need a keyboard, not the best one, but it has to have at least 3 working buttons and Mouse with both buttons working, if you have them, now just look for the enemy, Press right button and enjoy your kill in 2 seconds ;)