It is luffy as the captain and the rest, there is no hierarchy like vice captains or commander. Zoro might be the second strongest, but that does not make him the vice captain until it is stated in manga or sbs. It is like how katakuri is the second strongest of the bigmom pirates, but is not the vice captain
Some other characters like barto did say that zoro is VC.
Yes there is no hierarchy and everyone is treated equally but each SHs do have specific roles to play. Sanji is cook, Nami is navigator and so on.
But it's only zoro and luffy who have roles of being fighters.
And, zoro is the only SHs who has been hinted as VC by others like Barto. Since both Luffy and zoro share same role so being Captain and VC sets them apart as well.
And, every crew has right hand man and VC so one can cleary draw inference that SHs have it as well.
And, having VC doesn't create hierarchy per se because at the end of the day they all are nakamas.