Okay so, the HP and AP from Vlad's passive DOESN'T feed into itself, as in the AP Vlad gets for building HP will not give Vlad even more HP and vice versa. But, when you build Deathcap, the AP you get for building HP is affected by deathcap's passive, and the AP from deathcap's passive will grant HP, and THIS PARTICULAR interaction causes a positive feedback loop of mutual HP and AP growth, but it has diminishing returns and eventually levels off to zero (in reality it's an asymptote approaching zero). Back when cinderhulk was first introduced people found out it did the same thing, because it was a %hp increase based on the HP pool's own value just like deathcap was, and the same asymptotal shit applied. If you bought both Dcap AND cinderhulk it was even more significant since they'd interact with each other.
Overgrowth is a new source of bonus HP based on %max HP growth, just like cinderhulk was though much tinier. Have fun.