It's crystal clear that RHM and VC are two different things that occasionally overlay.
However while the VC is always also the RHM, it's not necessarily given that the RHM is also the VC and if that's the case the VC doesn't exist at all
Not for nothing when Rayleigh was introduced Oda had both titles in his info box "Former Vice Captain of Roger Pirates and Pirate's King RHM".
Marco's role for instance is Commander of the First Fleet but he is WB's RHM as stated in ch 909
However while VC is an official role in the crew, RHM is can be used for people who are second in command de facto like Zoro, Killer, Marco and so on.
Zoro is Luffy's RHM.
No.2 that becomes the captain's shadow and supports the crew !!