Does anybody know if armor outfits provide protection? Or is it just their stat buffs with different artwork and name?

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Evidently, some of them bump your Endurance and Strength, if that's what you mean by "protection". But other than that, the difference between them and, say, leather armor is a difference of degree, not a difference of kind.

I don't think they offer any kind of actual protection. I've never noticed any difference in a dwellers resilience or resistance to damage in combat with different armor types. I think they're basically just about aesthetics and SPECIAL buffs.

There doesn't seem to be any damage mitigation. Doesn't matter if they're wearing power armor, or just a vault suit. The amount of health the dweller has is all that seems to matter, as far as survivability goes.

(I should say, aside from pets which provide damage resistance. I did find a parrot with that bonus.)