when your vit bar is full, you'll see a red circle appear when you attack. You have to click as it's about to disappear. Too early or too late and you won't get it. If you want to practice the timing more, they have it in one of the training things.
And yea, the harder modes have the damn enemies running twice as far as they do in normal mode. Where the hell is my new game+?!?!?!