No official Vice-Captain or Divisional Commander. It is assumed that Zoro is second in command, but considering the everyone does what Nami says out of fear, that she could be considered second. They do not need a divisional commander since they have only one ship. 1) a fishman, likely to happen soon. 2) a pet, possible the kraken Luffy just tamed. 3) A logia type devil fruit user. The only type left for them. They need a Helmsman, Cabin Boy, Scientist (Franky is already a developer, so...) and a Fishman (Please, Jinbe)