Originally posted by Karma Chimera:

Do secondary hits trigger on hit effects?
Let's say you have a 2h weapon with vampirism that steals +10 hp per hit, and you swing it through two targets. Do you get 10 hp or 20 hp? Or what if you have the skill Shred Armor, giving 5 armor per hit, and you swing a melee weapon through two enemies- do you get 5 armor or 10 armor?

Finally, what about critical strikes and secondary targets? Do secondary targets even receive increased critical damage? Do you roll crit chance separately for each target, allowing multiple crits per attack, or only once for the primary target, applying increased damage to secondary targets based on the crit the primary target received? Can a skill like Blood Hunger trigger more than once in a single Frenzy attack that swings through more than one target?

Here's two Guides: Val's Guide includes info on Secondary Damage rules.
Armis / Chthon's Guide gives info on damage order in general.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=251337108
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3176679477

Difficult to answer this however from what I've observed:
Every target counts as a separate instance hence adds a greater amount per target struck for Shred Armor or another chance to roll an individual Crit on the second target.

I don't know if anyone has proven whether the Secondary Modifier interacts with the Critical amount.

I've seen a main target get a Critical and 2nd targets without Criticals.
So the Criticals seem to be a separate roll from the initial Secondary Damage event.
Getting a Critical on the main target has no guarantee to Crit on the 2nd targets.

I've seen Health Steal from both Blood Hunger and Equipment activate from all targets struck. I don't know the detailed interaction aside from that.