If anything, that just shows that Usopp talks big and Luffy is just an idiot for agreeing. Usopp is practically a coward. How can he be VC when he runs away and is scared everytime he runs into danger? Zoro would be much better running the crew than Usopp.
Luffy seemed pretty serious to me, looking for a cook and discussing the roles of the ship emphasized the importantness of the conversation despite of how laid back it seemed to have been.
And of course, I can argue Sanji would be just as good if not better as VC to his superior intelligence but would that make the closest to being A VC? Clearly not. Zoro while possessing the required brute strength isnt nessecacirly better than usopp at leading or even decision making, perhaps zoro is more level headed byt really aside from that and his strenght what else puts him clearly above the others? And please don't say loyalty, all of the strawhats were ready to die for luffy in thriller park. Also being a coward never, and I mean never, had prevented usopp from doing his duty in the crew or made him disobey luffy, thus while his character may be a coward his actions are clearly nit that of one when shit goes down.
Overall I'm not saying anything direct, just sharing the fact that luffy gave him the permission to handle the job of what seems like a vice captain duty, yall can draw yall own conclusion.
My original take was that most of the strawhats had acted like a vice captain at least once, thus that can be interpreted in two ways
1-they have no VC
2- They all share the duty of it unconsciously and oda did it on purpose