Thats the point, he doesn't want to accept it.
He wants to be known as Black Leg Sanji, and not Vinsmoke Sanji or Stealth Black (he even created a new name for it), or using his birthright powers to get stronger and changing who he is, he already made that pretty clear more than once that he prefers death in this case.
The only way i could see "Sanji accepting himself as a Vinsmoke" happening is with a huge drop of lore about the Vinsmokes, something pretty different than what he knows about Germa, and that possibly involves Sora so Sanji would be really interested at learning more about it, but that would need to happen on his fight against Queen.
