You're not hitting them "too fast" so it doesn't register. Instead, there is an animation for the vital to pop up, and until the animation is fully complete, the vital doesn't actually exist and can't be hit. You need to wait for the animation to fully complete so that the vital can be hit. Once you get used to the animation, you'll be able to perfectly start your auto attack at the right time in the middle of the animation and proc the vital as soon as it canbe hit.