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.