Naw... hehe
I'm not kidding. Sanji is best with no df power. The crews are gonna need his ability in swimming since he's the fastest among them. Besides, Sanji's pretty strong, enough to be in a row as the Monster Trio in Strawhats. If you noticed, in lots of arcs, Sanji would be separated with Zoro and Luffy, but he would never be seen alone, by his side it must be another nakama stuck with him, especially the one that least in physical strength. For instance, checked them out during Punk Hazard, Fishmen Island (though Luffy was with him, almost, but we knew it was for plotwise), Alabasta (remember when Luffy, Chopper & Zoro got lost? Sanji was with Nami & co.), Skypiea.
And even if he was alone, separated from the others, we would see him carrying out his own strategy, which benefit Strawhats a lot later on. Examples are during Whiskey Peak (his infamous nickname 'Mr.Prince' and 'shitty restaurant' calling with Mr.0; our Sir Crocodile. Because he interfered, Sir Croc lowered his guard about other possible threat like Strawhats, enabled them to arrive Alabasta Kingdom a bit easier without immediately being killed.) and another arc is Water 7 - Ennies Lobby. He stopped his rubber captain from saying something stupid to Usopp, he took care of Chopper, he chased after Robin alone, he closed the Gate of Justice for the sake of saving the whole crew... While facing all the incoming risks and dangers by himself with no aid, and not completely depended on other nakama to settle things.
Yup, he's strong already, just need to be more and more through the journey. He can be like Rayleigh, whom had no df ability, but a great swimmer, and developed CoC. In Sanji's case, maybe not CoC, but it's nothing impossible for him to fully master CoA & CoO. Though we all comprehend Zoro's still the best candidate as Luffy's vice captain.
