Oda confirmed it in recent SBS, so we can finally stop the debate whether Zoro is the Vice Captain or not.
SBS Volume 102:
Question: Hello Odacchi, judging from the color spread of chapter 1031, Bepo is the No.2 of Heart Pirates?! I was surprised since I thought him, Penguin, and Shachi are all No.2s. So is Bepo (Sulong form?) the strongest out of them?
Oda: Talking about that color spread huh. Same goes for Zoro, not everyone on there holds the position of Vice Captain. They are just the No.2s that I picked out. Penguin and Shachi are more reliable in normal circumstances, but those two have seen Bepo’s Sulong transformation, so they acknowledge that “combat wise Bepo is superior”!
These are just 2nd strongest in the crew, so Oda finally confirms that Zoro is 2nd strongest, which was kinda obvious.