There's no Vice Captain but if there was one it would probably be Zoro. Doesn't matter since Zoro, Jimbei and Sanji have different roles in regards to the crew. Zoro keeps Luffy in check and occasionally takes control. Sanji protects, comforts, advises and and on one occasion lead part of the crew. Jimbei protected and consoled Luffy when he was at his lowest and like the other 2 will do anything to protect Luffy. I also feel like mentioning Nami as she does her best to keep the entire crew in check as often as she can. At the end of the day, it's up to Luffy to decide and after +1000 chapters, he still hasn't named anyone his Vice Captain (I don't take non-crew members words into consideration as they don't understand the hierarchy, or lack there of, in the Straw Hat crew). As far as a hierarchy goes though, I take Sanji's speech to Usopp as a perfect description of the crew's dynamic. The one where he tells him that he'll do anything Usopp can't do and that Usopp should do what he can do. That's how I see it, everyone needs each other as they all have different skills but obviously one or two members aside from the captain will take charge in case of his absence.