The official Vivre Card dubs Sanji as the cook of the Straw Hat Pirates, who also sometimes acts as a “military advisor” of the crew.
This is my take on it. Ever since Alabasta and further, Sanji has often been the character who ends up acting independently in the favor of crew- specially if the Captain and others aren’t present. Some notable examples are the Mr. Prince persona and his actions during the Water 7. He also comes up with plans and better course of action for the group. A strategist. A military advisor who can come up with proper and effective tactics- who can decide whichever path will result in best results and what one should do in puzzling and complex situations.
However, strategy and plannings don’t particularly translate to being the leader- or rather, taking the charge/command of the crew during Captain’s absence or in Captain’s stead. Remember that every single StrawHat has done what Sanji does more often- Zoro sometimes coming up with plans, such as the ‘X’ mark during Alabasta arc, Nami doing so on several occasion and so on. It’s about who is smart enough to quickly formulate an effective strategy, and be the first one to suggest and voice it out.
There’s no official member who actively holds the position with the perk of “If Luffy’s not around, follow my orders.”, much like how Sanji’s not the stone-set strategist- however before I get to why Zoro fills in the role of former more often than not, a couple things to clarify:
- I do believe Sanji is better at emotionally encouraging others more than Zoro is- Sanji is expressive, and Sanji is kind. He’s encouraged other members of crew when they felt down quite often- more than anyone else. That’s a great leadership quality and I always agreed with the fact that Sanji makes an excellent leader.
- I also believe Sanji often takes the initiative and direct command of the group (regardless of whether it’s strategic or not) when both Luffy and Zoro are absent. Kind of like a Second Mate.
As for Zoro, the official Vivre Card notes him as the swordsman (fighter) of Straw Hat Pirates, and that he often acts as a Vice-Captain to the crew. I completely agree and here’s my reason as to why.
If he’s present, Zoro is nearly always the one who takes the initiative and actively orders the group to take certain actions. For example:
Here, Zoro stepped in, and he didn’t suggest what Luffy should do. It was actively keeping the Captain from making a bad decision, and taking the actual charge/command when needed. It displays in his dialogues (such as when he’d allow Usopp in, and how otherwise they are going to leave him behind).
Nami and Chopper were angry at first, reluctant a bit after, but while they did appear sad, they definitely didn’t seem like they were going to actively refuse it. They were merely silent afterward. Sanji agreed later on.