I wanted to remind people of some of the attitudes Sanji had as a crew leader and Luffy’s trusted man.
Every vice captain needs mature attitudes toward crew members to bring them to triumph and balance them emotionally.
Sanji’s attitude in telling Usopp that he can do what the other can’t do is very worthy for a Vice Captain.
Or when Sanji prevents the captain from making an unthinking statement and asking him to calm down.
When he organizes the team by explaining the situation they are in.
When he says to Jinbe the reason they must fight (when Jinbe disagrees with Luffy’s decision).
When he comforted Vivi.
When he questions the way Law is carrying out the plan (Law is the alliance member who is articulating the plan).
When it prevents Chopper from acting, protecting Usopp from a feeling of failure.
When Sanji next to Zoro, he also intends to sacrifice his life for Luffy, and Brook acknowledging their will.
-LEADERSHIP AND HIERARCHY
The vice captain must be a leader at various times and hierarchically above the other crew members, so that everyone respects them and follows their orders.
See how in the absence of the captain, Sanji leads the team to counterattack a Yonko ship (this is an excellent role for a captain’s deputy).
Also, when he leads a plan with Usopp and Franky on the ocean train.
Also, when Jinbe recognizes Sanji’s superior hierarchy, he apologizes to Sanji, and begins to report the entire situation they are in.
Some other times Sanji exercises leadership:
His strength is always framed with Luffy and Zoro, all of them being called “monsters” and being far superior to the rest of the pack.
When Zoro was injured in Arlong Park, Sanji took over Arlong’s right arm (showing that Sanji was decently able to defeat the enemy organization’s right arm).
Plus it’s totally debatable if he would be able to defeat Kaku and Daz Bones.
I don’t want to delve too much into Sanji’s strength, but I believe most people accept that Sanji is strong enough to be vice captain, even though the distance between him and Luffy is greater than Luffy for Zoro.
-CLOSE RELATIONSHIP OF TRUST AND RESPECT WITH THE CAPTAIN
To emphasize the long history relationship between Luffy and Sanji, I will start from the beginning when Luffy awoke Sanji, both had their idols who sacrificed a body member to save them.
Throughout history they have fought oftentimes together, creating a deeper bond:
Luffy and Sanji vs monster cow-fish
Luffy and Sanji vs Laphans
Luffy and Sanji vs Satori
Luffy and Sanji vs Wapol
See when Sanji was portrayed alongside Kid and Law’s Vice Captains.
-HOW LUFFY/SANJI RELATIONSHIP DEVELOPED IN WHOLE CAKE ISLAND
People often say that Sanji “cheated” on Luffy and so he is not as trustworthy as a vice captain would be. But this is absurdly false, there were a number of events that involved the integrity of people Sanji likes, and Sanji orchestrated a betrayal of Luffy just to protect the people he loves, like Zeff and the Mugiwaras, not to mention his hand of cook that could be blown up.
Contrary to what some think, Whole Cake Island has developed the deep relationship between Luffy and Sanji.
In this bow, Sanji was the only Mugiwara that Luffy says could not be king of the pirates without him:
Both also always trusted each other, at the meeting point for example, this in my opinion is an excellent form of relationship between Captain and Vice Captain.
Finally, Luffy and Sanji together repel the attack of a powerful Yonko.
Looking at all this, there is no way that Luffy and Sanji do not have a great relationship of Captain and Vice Captain, after all these interactions of trust and respect for each other.
Luffy has 2 right-hand men. These 2 are the pillars of the crew and the wings that make Luffy fly.
*Theory by Chaves