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Vergo's fit is white so it's much easier to show the contrast. Moreover not only does Sanji not only always wear dark colored clothes, he also coats his legs in flames which makes it that much harder to tell. I'm pretty sure there are some panels somewhere in Wano or on whole cake where he was either wearing white or a kimono that show him utilizing it but again I'm sure the argument will probably be it's too small to see 💀 been through this with y'all before and I honestly can't be arsed to go on that wild goose chase on what I think is an obtuse argument in the first place (no offense to you personally). Crazy to me that anyone that isn't being disingenuous would opt for the possibility of Sanji not being able to use an ability as common as armament hardening over the more logical argument I've presented above but I digress.
Not reading your blogpost.