User blog comment:Cdswalkthrough/Vinsmoke Sanji or Just Sanji/@comment-5386071-20180503021700/@comment-194.82.172.247-20180503111654

It is simple.

Unlike a tree which can only receive a name from external parties, Sanji can choose his own identifiers based on what he considers to be his identity.

If we believe that his ability to think rationally is affected and therefore cannot properly establish his identity on his own or that he is lying about what his actual identity is, then sure - call him Vinsmoke. Of course, unless you don't wish to respect his integrity as a (although fictional) sentient being.

Sanji is completely justified in disowning his identity as a Vinsmoke. He is the literal opposite of what his father established to be the group's identity (kind, soft, servient, humble as opposed to ruthless, hard-skinned/militaristic, served, proud). He has never talked, nor does he continue to want to talk about his past. He lied when telling Luffy that he is a Vinsmoke and Luffy caught onto that lie - fact which was confirmed when Sanji asked himself what the hell he is doing and ran to meet with Luffy at the promised spot. Sanji is not drunk, drugged, delusional, mentally ill or under somebody else's mental control or coercion when he affirms strongly that he does not want to have anything to do with that name (with the maximum of not even willing to hear the name said out loud in his presence).

He believes himself to be Sanji. He is justified to not be branded with the name of his family - both disowning and being disowned by his father, the legitimate leader of the group (i.e. his children are literally rendered physically unable to disobey him).

If the situation was ambiguous, then fine - it would be hard to tell. But in this case, it is unequivocally clear that he wants to be just Sanji.