I figured. Sanji could never be vice captain because he would prioritize Nami and Robin over everyone else in the crew. The only person Zoro listens to is Luffy, whereas Sanji has listened to Zoro before.
Oda really emphasizes and draws attention towards Sanji's kindness. A vice captain can't always be kind. Difficult decisions need to be made at times and sometimes you can't take into account someone's feelings in order to protect the whole crew. Luffys carefree attitude and Sanji's kindness may not be the best mix in terms of leadership style.
I’ve always read it as Sanji and Zoro signify two different types of leaders.
Sanji stands up for the crew even to Luffy. When Luffy was about to insult Usopp Sanji stepped in and stopped him. When Nami was talking about letting Usopp back without him apologizing to the Luffy and the crew he told her she was wrong. When the crew needs defending he’s always the first to step in line. He’s the crews guardian Angel.
Zoro ensures Luffy is held to the standard as a captain. When Luffy is joking around too much he’s there to tell him to take things more seriously. When Luffy isn’t being mature he’s there to knock sense into him. And when it comes to standing up to defend Luffy Zoro will jump in to do so. Zoro is Luffys personal monster who attacks who Luffy orders and makes sure Luffy is strong enough as a captain to ensure the monster doesn’t turn on him.
Both are vital to the straw hats and are excellent leaders in their own rights. I think Water 7 showed this better than any other arc. Except for Sabaody when Luffy Zoro and Sanji decided to separate and each take a group of straw hats with them.
People say right and left hand, people say chief of staff and vice captain all in all its always monster trio