When were those moments that he led the crew ??
Zoro led the crew after Enies Lobby when even his captain was about to make a huge mistake by running towards Ussop instead of acting like a real captain.
That moment made it clear that Zoro is the second guy in command when even his captain is obviously too bias to make proper decisions as the captain. Thus, Zoro was there to tell his captain that he has to pull his shit together because he's the effing captain.
The second moment is in Thriller Bark when Zoro was even willing to sacrifice his own life by protecting his captain and the crew.
Why do you think Zoro knocked Sanji out in the first place? It's because of Zoro's responsibility as the vice captain not to endanger his crew mates.
Heck, at first Sanji should demonstrate that he's
on par with Nami in command since Nami as the navigator has huge responsibilities in the crew. That's why she discussed with Zoro about Ussop instead of Sanji.
I have dozens of panels showing Sanji leading the group in the absence of Luffy (not saying that Sanji is VC), but Zoro is just a leader who looks higher, just like Sanji.
I have dozens of panels showing Zoro commanding and leading the crew even when Sanji was there.
Btw. Sanji hurting Luffy, disrespecting him and leaving the crew already made it clear that he is
not on par with Zoro in terms of hierarchy. Fuck, he's not even on par with Nami...