Oda basically confirmed what we already knew. Zoro is not officially the Vice Captain of the Straw Hats, and personally, I don't think he'll ever be. However, he serves the role of one in every sense, and guides the crew at times when Luffy cannot. Even characters in universe mistake him for one.
He is, at this moment, the only other Straw Hat besides Luffy to possess Conqueror's haki. And so far, most Conqueror's that we've met have either been the leaders of their group, or worked directly under another Conqueror. There is the Rocks pirates, where 3 confirmed Conqueror's worked under Xebec. Though he's not confirmed, it's a safe guess he possesses Conqueror's.
That leaves Sengoku as the only exception, but we've seen him act against the wishes of his superiors at times. And now he's semi retired, doing what he pleases.
In Zoro's case, he is a Conqueror to a T. Aside from his ambition to be the strongest, he refused to join any group or organization unless he was the boss.
Zoro is described by various characters as not a follower by nature. That someone like him should be the Captain, and possesses high standards for Luffy.
From a pure story perspective, Zoro as a subordinate is viewed as an anomaly, serving to highlight himself as well as Luffy for managing to recruit him. That is diminished if he's deferring to multiple people. And when coupled with that, along with his disposition as a King and leader, he has all the ingredients for most viable Vice Captain/First Mate candidate of the crew, even if it's in an unofficial capacity.