Usopp is the vice captain though. Not Zoro. Luffy literally even agreed to this way back. Also it wasn't Zoros place to challenge Usopp there. Usopp had more faith and trust in his crew than anyone, that's literally why he left, because the Merry was a member of their crew who shouldnt have been left behind so casually, and to him, if they could leave Merry behind, they could leave anyone else behind.
Zoro being so ready to drop him is proof he's not ready to be vice captain.