Like title says think Physical vulnerability should be changed to reduce 10% physical resistance per stack, some it actually feel useful compared to alternatives for higher difficulties. At 8 or 10lvl plus void crusade the resistance or damage soaking of enemies so high, even with buff like 50% damage for 1 mission simply helps nothing, may be unaware of this but have found no item that actually reduces enemy physical resistance unlike heat for instance that get their -50% heat.

Problem I feel is no matter how high you raise physical damage it would need to be in the absurd to matter against extremely high reductions, while the Armor Breaking can reduce part armor mitigations, the vulnerability only offers more 10% damage, which will become moot if its highly reduced from resistances.

Lets take 5000 damage just as example.

If you add extra 50% damage it will reach 7500 damage, but then on other hand if that 7500 damage is substracted by 50% damage reduction from physical resistance actual damage going through would be closer to just 3750 damage, which effectively make 50% from 5 stacks of vulnerability useless in comparison to high resistance reductions, since we can't lower any physical resistance far that I can tell, compare to dealing with armors where we can use either piercing or armor break to at least get by some it.

Another example is when I do normal map with no changes in damage migitation from enemies, I roughly hit for 4500 damage or something, when I do lvl 7 plus or something damage down to roughly 1440 or 1500 using armor breaking or physical vulnerability or not.

It can be intention that to do higher difficulty crusade with physical weapon builts that option to complete them is not to use them. If so I think is really bad design decision.