Oda will never give a definitive answer to Zoro vs Sanji because…it doesn’t matter at all. They are never going to fight each other. Their opponents will always be on the same tier.
Their feats will remain the same level. Zoro cuts a mountain. Sanji kicks a mountain. Zoro cuts a tower. Sanji plays Jenga with it. Zoro cuts the Kraken, Sanji toasts it.
There’s the argument that Sanji’s strength devalues Zoro because he wants to be the greatest swordsman. Sanji’s own strength is him being one of the world’s greatest martial artists. He was trained by Zeff. Overcame Fishman Karate, Okama Kenpo, Tekkai Kenpo and then 99 masters of Newkama Kenpo. Acting like Sanji doesn’t care about his martial arts prowess or that he doesn’t train is odd since we literally see him group against and crush all these martial artists. Oh and Sanji’s first chapter was about how cooks are the strongest on the sea.
Yes the Vivre card talks about how Zoro’s strength is second to none, but it also talks about Sanji’s being top-class.
The only rational position, based on the manga, is Zoro >= Sanji. I would put Zoro > Sanji, but the manga keeps pushing the Zoro = Sanji thing whenever the two are directly compared to each other.
As for Oda’s only actual answer on this topic:
D: “If Zoro and Sanji had a serious fight, who would win? Zoro beat Mr. 1, so… he’s stronger, right…?”
Oda: “Whoa! That was surprising… I thought you said “If Zoro and Sanji had a dick fight”…You have to be careful what you say. Someone might hear something totally wrong… Okay, see you next time!!”
However, in the end, it doesn’t matter, doesn’t change anything about the story.
*Theory by GabrielMichaelson