The Vayne buff

Even with her feed that she's got from roaming mid lane,I find it very unlikely that by 25 min lv 15 Vayne would crit my lv 13 Jhin with one Q+ autoattack for 90% of his hp when he was full hp. What could he do? His trap couldn't do anything,he missed his W skillshot and tried to run away,but he couldn't be dodge point and click adventurist so I had to flash devour his ass and let my team gangbang Vayne. Why is she allowed to deal so much burst damage? 70% total adx250% + 250% ad for on 2 seconds cooldown for someone who suppost to right click you over time?