Start building Vlad AP after the patch.
The only advantages bruiser has is rocket belt and ~500 HP, essentially. Considering AP already has 3500 HP, its not a huge deal. That's not factoring in strength of the ages, mind you.
Also consider that AP's healing will be a significant amount higher due to Vampirism, provided you put 9 in offense, along with spending less HP and healing it back up easier.
:
bonus HP: ~1500 (~3500 total) AP: ~750 with blue, 675 without bonus MR 60 bonus armor: 50
CDR runes required
:
bonus HP: ~2000 (~4000 total) AP: ~460 with blue, 400 without bonus MR 55 bonus armor: 50 bonus healing: 20%
CDR runes optional
Abilities (without blue)
AP
Q damage/heal: 506 / 276
CR Q damage/heal: 1012 / 476+ 560 at 50% HP.
E: 414-1065
R damage/heal: 823 / 823-2468
Bruiser
Q damage/heal: 355 / 216
CR Q damage/heal: 710 / 416+ 504 at 50% HP.
E: 330-820
R damage/heal: 630 / 756-2268
Rocketbelt damage: 290 +20%/bolt
As you can see, the only thing bruiser build holds over full AP is rocket belt, not needing 10-20% CDR from a runepage (depending on boots), and ~500 HP. The HP is essentially invalidated by the advantages of the AP outside of pure burst situations against you, which don't happen unless you messed up really badly.
Bruiser comes close at 3 items, due to 3 HP items in a row, the increased sustain, and rocketbelt's damage, but past that AP is ahead 100% outside of the mini-dash from protobelt.