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Vergo ankle vs Sanji’s shin (weakest part of the leg vs Sanji’s strongest and hardest part of the leg)
Vergo is attacking Sanji with his ankle foot, and Sanji is checking high up on his knee joint. Sanji’s shin bone is the strongest at this exact point; the leg bones and ligaments connect here, the connective tissues and muscles are thicker here. Vergo breaks Sanji’s fibula and almost snaps it in two when he is clashing with his ankle.
Vergo is using minimum range of motion with his leg bent and he is using the weakest not eve his shin in the clash but actually clashig with his ankle which has weak muscles and weak ass bones, all very susceptible to trauma. The way in which Vergo attacks is the last thing you want to do because an ankle joint injury is as good as certain. We know Sanji even has hardening as he did use it against Drake (latest anime confirms) and we can be as good as certain he used it. Even Luffy mentions in the same arc that Sanji is part of the three in the crew capable of using armament haki, meaning he most assuredly used armament haki against Vergo.
When Vergo runs up on Sanji he is seen lifting his left leg sort of feigning attacking with a left roundhouse kick, as here you can see he is actually clashing with his right leg. That means he switched position, traded momentum and power just for a leg switch meant to mess with Sanji's head. Vergo switched legs so fast Sanji can't barely even react to it (hence the three exclamation marks).
Just for reference, just look at the most famous and iconic leg break in UFC history that is almost completely identical to the one featuring Sanji vs Vergo, only the result was the complete opposite: i.e. Anderson Silvia vs Wiedman. Here Silvia loses for the first time in 7 years because he does the same “mistake” Vergo does kicking with a low angle of his foot and Weidman checking high up, just like Sanji. Silvia is even lucky he didn’t kick as low as Vergo because an ankle injury would have been far worse. Just ask Silvia who needed surgeries, titanium implants, 6 months to recover and started taking steroids to dull the pain
Vergo should technically break worse than Silvia here, yet instead he is the one that cleaves Sanji’s bone almost in two.
Sanji got so badly humiliated by Vergo it is not even funny, though it actually is very funny