That's why I said the databook is outdated. The story developed in a way that proved it to be wrong information. The same way that the crew's dynamic evolved as the story went on to the point where there is no hierarchy. At the time, Zoro was the closest thing to being the VC, but now, the crew's dynamic is at the point where no one is like the vice-captain.
Zoro never threatened to take over as captain. He jokingly told Luffy they'd need a different captain if he couldn't knock out as many Fishmen as he did, and then told Luffy later to simply takes things more seriously. Now you're just making things up. Sanji also said "He's so tough on himself," not on Luffy.