I think the biggest difference between Jax/Kayle and Vlad is that the former two are auto attack based champions. Also, just to establish this, Vlad has had his passive nerfed a few times, and at this point its only worth it to build AP and get hp from it; very rarely is the passive used the other way (and if it is, the AP he is receiving is minor).
Now, because vlad's damage is limited by cool downs, he doesn't steam roll over champions as easily as champions like Jax and Kayle do. I don't mean to say that Vlad is particularly well balanced, but when Jax built his auto attack damage (which is on a what, 1 second cooldown?), got sustain, and damage stats all at the same time, he was quickly becoming impossible to deal with. Vlad, on the other hand, has to go through multiple QE rotations to wear an opponent down (bear in mind that doing so would also push the lane).
Bottom line is I think Kayle and Jax lost their free stat passives because it made them too effective at controlling lanes due to their play styles. Vlad doesn't control the lane the same way these champions do, and as a result, has been allowed to keep his.