Usually I'm not the biggest fan of memory loss AUs but the thought of Big Mom forcing/manipulating Pudding into erasing Sanji's memories of the crew and editing them to make him believe he wants to be a Vinsmoke... Luffy desperately trying to make him remember but Sanji genuinely not being able to even recognize Nami...
Sanji still maintains his heart and little traits of himself even if he believes he's part of that family. Even if he acts like them. Even if he has a different personality. His heart is still there, and Luffy knows. "Say you're Vinsmoke Sanji all you want, but I know the Strawhat's cook- My cook is still in there" Luffy is determined to make him come back. "I- I don't know who you're talking about" But Sanji is hesitating. And Luffy knows he's in there.
So, to summarize, thanks to the power of love (great logic Sanji's character always uses so I'm using it here too) Luffy manages to make Sanji remember without even needing Pudding because their romance can break any Devil Fruit's ability. Because souls and love are connected to memories and Luffy and Sanji's bond is enough to make them reconnect again.
