Nami bosses them around on the sea because she knows the best way to navigate the ship cause she’s a navigator.
Zoro and Sanji are the wings of the pirate king and the two leaders under Luffy. That’s why they lead groups when Luffys not around and not Usopp or Nami.
Trying to put Usopp as the vice captain and Nami as the chief of staff, two positions which mean the number two on the ship, is just downplaying Zoro and Sanjis importance because you don’t like it.
		
		
	 
 Again it’s not only when navigating
Nami is usually in charge bossing them around unless there is a serious combat related threat in which they are more suited to take charge
Again mainly when it comes to combat related danger 
In general not really 
The strawhats aren’t your typical crew 
At the end of the day with or without those positions no one is straight up in charge of anyone else. Tho in general they do listen to nami a lot 
And usopp being vice captain won’t mean he’ll boss around Jinbe