I don't understand this difference between AP and AD items.

I tried out Vlad with the new runes and half way through the game I realised I don't have my 20% cdr/lvl anymore. I have to go domination/precision for heals and dmg. So my question is, am I supposed to buy just to get 40% cdr and end up with 200 AP and little dmg after all those items ?

Then I watched Zed buying . Literally every time has cdr and everything you need. AD + lethality + even hp + mini ghostblade active + extra dmg when unseen + armor pen, etc... Even when they screw you over with hexdrinker you can build it into with 10% cdr also. Oh yeah, , just for the lols it has 10% cdr also.

Same with ADC's. They don't need cdr that much so they have all the AS, AD built in cheap items, except IE. Even if you need cdr on an ADC, you can get reaver and get 30% cdr just like that. All that while still having dmg, AS, AD.

Then you check AP items, and other than morello, which I cannot use on Vlad, all have built in decisions into items. If you want ap, well you won't get cdr and vice versa. But AD items all have everything.