In theory, the Captain and the Vice-Captain should be at a combat power nearly equal because the Vice-Captain must be able to become Captain if the Captain dies, is sick, is really tired, is absent etc ... He must be able to act as a Captain if the true Captain cannot take on his position at a moment which means all the responsabilities of the crew go to the Vice-Captain (for example, be able to protect all the nakamas etc...)
The Vice-Captain of the Pirate King, Silver Rayleigh was really strong (he is still able to fight against an admiral, to use a wonderful haki), so I guess he was near the Pirate King level.
So, the Captain and the Vice-Captain are able to fight against each other. If Rayleigh is still able to stand against an admiral, he was able to stand in front of Gol Roger because he was stronger in the past.
Moreover, I think an admiral can fight with the Pirate King even if he would be for sure the looser.
Plus, as we know, the two guys that know they always have to get stronger are Luffy and Zoro. So, I think, they've really got the same will to grow, and by consequence, they should be in a level relatively equal. The Captain by definition is the stronger guy because he had all the responsibilities. But, if the Captain and Vice-Captain fights, there are no position anymore. The two fights because they cannot stay in that hierarchy. For the moment, they're still in that hierarchy but Zoro has already intervened in the Captain's (Luffy) decisions (at W7 for exemple). So, for me, they're still equal and Luffy cannot be the winner just because he is officially the Captain (unofficially, he is always challenged by Zoro even if they both don't want it).
There is a time he will be ruling over his crew and his decisions will not be challenged by the Vice-Captain : it will be when they'll be at their top level of their life, when Luffy will be able to fight/decide/"rule" whatever the other people think, at the time he will be able to fight against all the kind of combatants
.
But for the moment, he is not able to do it and he is (as it will always be) near his Vice-Captain level. So, I think, as a swordman, Zoro would win because of his advantage.
while reading your post i found myself agreeing and disagreeing
As you Say captain and vice captain should be close in strength and your example of gold rodger n silver rayleigh is perfect example
If silver can handle admirals at an old age imagine what the vice captain of the future pirate king would be doing
But i disagree with your view on luffy not leading yet until later
in fact he has been leading he why do the crew just go to every island and explore and have fun its cause thats what luffy wants
most of the battles they enter since they arrived in the new world is due to luffy leading them into the situation
arabasta (he chose to help)
Ennies lobby (he chose to fight)
Tb (he chose to fight)
jaya arc (he chose to get beating with Zoro listening to him even after nami said fight)
Sky island (he could have escape chose otherwise too stay n fight)
Zoro follows luffy even if it could lead him to his death thats because every fight luffy leads the crew into it helpsmake them stronger
Zoro only challenges and controls when things are serious and luffy either being too dumb or too mushy
Like with ussop, Zoro wouldn't allow him back and Luffy accepted his view as his vice captain and the reason Zoro did that was because he didn't like the way Ussop tryd to disrespect his captain
Anyone disrespects luffy, Zoro will be there as a vice captain
it makes the two of them quite cool to watch just as the supernova said luffy must be a great man to have someone like Zoro serve under him
yea they are two guys with the highest goals but they respect each other
as i see them they are two sides to a coin
Captain side - luffy - joker free willed not really serious at the most of times
Then the other side
First mate - Zoro does his training sleeps drinks and keeps to himelf in the most very opposite to luffy apart from the ambition that drives the two of them
but when Zoro sees that luffys doing something that would endanger everyone or when he sees something he finds wrong then He puts his foot down, luffy awknowledges Zoro and for the most part Zoro is right
I for one thought he was making good sense when he said that ussop shouldn't return unless he said sorry first, tho ussop didn't do that and they just ignored him til he did, but just having him appologise was enough, if he didn't they would have left him in w7 due to Zoro no doubt about it
Luffy listen to Zoro as a first mate but i Luffy makes the descisions and Zoro goes along with them as a first mate should
and yes Zoro and luffy should stay close in strength especially if they will fight admirals
cause even if luffy became pirate king he would not be able to defeat everyone if they thought him at once, to be pirate king he needs a crew that will not loose to any other crew and to do that he needs both Sanji and Zoro to be at or near his level of strength
3 admirals - a nice number for the strawhat crew luffy zoro and sanji by the end should be at or above this level
7 shichibukai - Luffy fought 2, Zoro's fought 2 tho the out comes were opposite
4 yonkou - well really its probably 2 luffy will fight but then remember these guys will have their own vice captains etc
even some of the supernova
1 already probably will try to take out Zoro or luffy (bonney)
then you got the others who are a nice mix of swordsmen captains (who could fight Zoro) and Captains for luffy to battle against aint it funny that Oda kept Zoro in the rankings with the other supernova, Oda never does anything for no reason
and with Luffy gone stupidly alone into the the enemies mouth, Zoro may have to lead the crew into ID to help get him out if luffy manages to get ace, as i think gets himself caught.
dang everytime i wanna finish i think of something new lol
well last point
Oda didn't brink in Silver for no reason
I believe he did so because he wanted to develop Zoro
Sliver and Zoro's comparriosons
both like to drink
both are serious (well judging from silvers flashbacks)
Both seem to be swordsmen
I can easily see Oda having a Zoro and silver few page or chapter chat where they either talk about Zoro's previous sacrifice for his captain or maybe brookes sanji or robin tell silver about Zoro thinking he's still not strong enough to lead the crew (maybe because Zoro plans to help luffy if he got himself caught) then the two have a 1 on 1.
im gonna stop typing ^__^