The general notion of the "vice-captain" is the second in command someone who is capable to take the helm when the captain is not present (Imo its zoro I'll give my perspective at the end) . In the case of Straw hat Pirates
its a bit intrinsic than one thinks it really is, from a direct sight one assumes that Zoro is the vice captain coz he is the first mate, but which gets negated since we know oda planned on keeping ussop as a vise captain,
even the viver card states that zoro has "acted" like the vise captain in certain scenarios as a voice of reason like in water 7 reminding luffy who he is and makes him realize the pedestal he stands upon is that of a pivotal head of thr crew,but it doesn't really emphatically points out that he is the second in command ir the vice captain.. but if that's how you determined who's the vice captain then.. you have another solid example as sanji.
Now here is the thing, sanji also has played the role of being incentive of morale for others like Vivi in Alabasta and ussop in Enies lobby, And and also led a part of the crew to a separate expedition but does that mean that he factually is the Vice captain? both zoro and sanji have done alot for their crews
Theyy both lead the vanguard of the crew into battles.. if the Cap is not present..like in the case of thriller bark, they both have even protected the crew by going on out alone and defending it, in zoro's case it was Whisky peaks and in sanji's was the whole alabasta arc and enies lobby and have sacrificed themselves for the crew and their captain...it doesn't end here, now in comes jimbe. In WCI jimbe was in charge of the whole crew when Luffy was in midst of his fight with katakuri and sanji gone to stop BM's hunger pangs, not only that but he even raised the crew's morale after Pedro's death, just like zoro and sanji did, they 3 have striking parallels and well suited characteristics to become the Vice captain..
But here's this catch with sanji, unlike the other 2 he goes off and does his thing to protect the crew but in certain situations you can rely on him as the Command center of crew at hand like The Twirly pirates in Zou..and that was a rare occasion as well...if it was zoro instead of sanji he'd do the same if he understood law's plan.
I think we can come to a conclusion that its very very circumstantial that each if them become the Vice captain of their crew when luffy isn't there , and coincidentally if sanji is in charge zoro isn't there, and if zoro is then sanji isn't there, and if both of them are there then they both usually do their own thing but collaborate when it comes down to it, like in thriller bark... and when both of them aren't there then jimbe becomes the command center.
But Imo even if you "can" come to the conclusion above... portrayal wise Zoro takes the role.. he has been with luffy through thick and thin even more than sanji has been where sanji goes his own way to protect the crew. I just read thriller bark and noticed zoro was more worried about luffy's shadow than his own and everyone agreed waiting for luffy to come back is a gamble this just means that the captain is first priority no matter what and that notion of staying with the Captain! portrayal wise the story SHOUTS that zoro is the 2nd in command.. the reason why zoro is there on the roof top is due to him being by Luffy in the light luffy has been in and obviously is parallel with Killer.. who is only another Vice cap in in the SN's. Ifzoro isn't available one can always count on sanji's expertise to guide them the same goes with jimbe... just like kaido has 3 right hand men with king taking the highest command.. you can comparatively say the same for zoro, sanji, and jimbe, so if luffy had to split his crew he'd have these 3 as the heads of each respective team.
Btw thriller bark is a must read.
*edit : this is my opinion should be obvious
