As others have said, there are two critical figures:
The size of the capacitor - the amount of energy it contains, and
The rate it recharges - this is normally given as “seconds”, how much time it takes to refill the capacitor if nothing is drawing on it.
Both are improved/modified by skills, modules and rigs.
The Vagabond has a smaller but faster recharging capacitor - it can withstanding more demand on it than the larger much slower recharging capacitor in the Command Ship. Makes sense since a HAC is going to be actively trading blows, manoeuvring quickly and living a high energy existence - much more than a Command Ship that is intended for a much more support focused role that doesn’t have as greater capacitor need.
The Sleipnir may have twice the capacitor but it’s more that twice the recharge time - in terms of sustainable GJ/sec it’s thus less than the Vagabond
How you fit each depends on the role you have for it. But a few thoughts.
Rate is “to fill the capacitor” adding size thus increases the recharge rate. That large battery is doubling the capacitor size of a HAC which doubles the GJ/sec that can drawn from it. It’s only adding 50% or so to the Command Ship and only thus increasing the already worse GJ/sec by that proportion.
Smaller batteries in big hulls are normally not worth it, the Cap Rechargers give you a better chance of stability since they are a percentage improvement.
Cap Boosters have a role, a quick slug of power to give you chance to act when the capacitor is under high demand can be extremely useful, but for long missions it’s not a good route to stability.
Capacitor rigs are percent based, so for big capacitors they can provide quite a bonus.
Recharge rate mods stack, play with it but mixing capacitor size modules with recharge modules may help a bit. Or not.
Passive shield recharge rate is often improved at the cost of capacitor performance.
MWDs have a capacitor penalty in the small print.
Stability is over rated, even mission running. You won’t need everything running at once, learning to manage demand is as important as learning how to manage incoming damage. In combat against another Capsuleer stability isn’t needed - if the battle only lasts two minutes then why waste space on needless capacitor?
Most combat pilots will carry a Neutralizer in a spare high power slot. Being on the recieving end isn’t nice. Fortunately they are short range (though some Amarrian ships have nice cap.warfare range bonuses).
I normally fly Amarrian ships. Forget the Vagabond, I first flew Sacrileges. A dual rep, large battery MWD HAM Sac. is a lot of fun and almost stable (more stable than the pilot!)
In general Amarrian ships have better capacitors, but they are normally needed for powering energy turrets.