Capacitor skills- train them.
They may seem like a waste of time vs training skills that will allow you to fly more powerful ships, or use better modules- but core capacitor and fitting skills apply to every ship you will ever fly.
If you feel like the capacitor on the vexor is uninspiring you are likely to feel that way flying any other ship as well.
Lastly, as people before have mentioned, if you are going to active shield tank, don’t use a shield extender. Instead, increase your shield resistances vs the rats you will be fighting, which greatly amplifies the effective repair rate of your shield booster, so much so that you should only have to cycle it a few times occasionally to keep up with incoming damage.
You generally have to sacrifice something to have a cap-stable active shield boat- trading damage, application, or mobility to do so. Cap stability is a bit of a red herring to chase for missioning, anyhow. Don’t aggro the entire pocket and fly intelligently and you don’t need much more than 3 min of sustained repping.