The way I see it, his whole beef with his family simply isn't on the same tier as his stance regarding cooking, not using his hands to fight, and his inability to hurt women. Those are iron-clad rules that we know Sanji would never, ever break, that we've associated with his character for over a decade.
The Vinsmoke stuff on the other hand, that's pretty much brand new in comparison. And for all his justified resentment, he still went out of his way to save them. You already have him having to face the fact that his bounty will now always carry the Vinsmoke name. You have the raid suit that only he can use, introduced in a way where Oda makes it obvious that it'll inevitably be used in some form. And said suit was surprisingly given to him by Niji, hinting that though his family never admit it anytime soon, it's their way of saying they now acknowledge him.
If I were to just look at his new bounty, IMO that alone would be enough in my mind to suggest that Oda has every intention of getting Sanji to a place where he begrudingly accepts that he's a Vinsmoke. And if that indeed is the case, then the only question left afterwards is just how far Oda plans on going with said acceptance, namely whether or not he'll put on the suit as well. And I think he will.
I feel that if Oda really wanted us to consider the possibility of Usopp and/or Franky messing around with it, he wouldn't have bothered to include the detail of it only opening for Sanji. Because otherwise, why bother to point that out, only to end up giving the suit to someone else later on anyways?