The problem with that item is that they are either mutually implicit (if the passives can be stacked, which they can't) or mathematically metter than the other. In your case, if the target you attack has 86 or more armor, it's better to use Peacemaker, right until ~15 armor where they most probably break even (depends on runes and masteries). Once you factor in Armor runes, most of the champions have more than 86 armor at level 18. Otherwise, any single tier 2 armor item will bump all champions except a soulless Thresh over the 86 cap. This means that overall, your item is a mid-game item
After that, in what case do you use the item? You pointed it yourself. This item is preferable if you can zero in on the enemy quishies while disregarding tanks. The only ones truly able to do that (through AD damage) are assassins. So what you built is a mid-game AD assassin item. It arguably works for AD casters too, but later on they will be getting run over by bruisers, so it's not as useful.
The other case where you can aim for someone with low Armor is pretty straightforward : early on. The earlier in the game you are, the more potent this item is. At 40 armor, you're dealing almost true damage. And when does assassins become a problem? Mid game when they first get access to their ultimate, and whenever they get ahead of the pack. This would mean that you could rush your brutalizer with no drawbacks (usually it means not getting that T3+ item early, since BC isn't a good rush in most cases), and then if you get ahead, get that early Peacemaker for infinite 100-0s.
All in all, this item can't exist, because it'll either be so bad that there's no reason to use it unless you're in that ione in a thousand case, or because you're basically letting all mid-game AD champions (Zed, Renekton, Talon, Pantheon, Draven) single-handedly fight the game between themselves.
Another place where this design is fundamentally flawed is that as-is, you can stack peacemaker and brutalizer, which makes little sense. If you apply vanguard to brutalizer, yo uhave to apply it to BC and YG too, and in that case you're ruining a few champions' itemization (Youmuu's + LW is seen quite a bit, BC + Lw sometimes).