VoS really works for those overused miracles/spells like WoG and dark magic, but I remember when on Demon's Souls I would always use a soulmass or weapon buff and THEN use the silencing miracle, since they could not be ACTIVATED once it is in effect. This might apply to Dark Souls, too. (Not sure if applies to karmic justice's activation after being attacked)
As for Karmic Justice, it can be very useful with the proper build/stat management. It activates when you receive 5 damages to your HP within a short time, whether directly or against your shield that doesn't block 100% dmg.(poison, toxic, power within, chaos blade don't contribute) After receiving the hint of how karmic justice gets its damage scaling from the main-hand talisman/catalyst/pyro glove's magic adjust, I did some experimenting and research on it myself.
From my tests I found that this is indeed true, so the ones with most mag adj (tin crystal catalyst for high INT, ascended pyro flame +5 for no stat towards karmic justice damage) will do the greatest damage output with karmic justice. Be sure to have it equipped and currently out in the RIGHT hand when Karmic Justice makes the explosion.
However, if you can't stand not being able to fight with your weapon while waiting for karmic justice to activate, then there is an alternative. I found out that the scaled-on magic damage (from Int or Faith, including dragonslayer's lightning scaled damage), which is not the base magic damage that the weapon has regardless of stats, acts like mag adj in a similar but not exact way.
---With weapons it tends give karmic justice about 200-230 damage with no magic dmg scaled on, but when it does have scaled on mag dmg it takes on a very slightly worse value of mag adj, with the strange divine and occult 110, 120, etc. numbers amplifying it by an amount unknown to me (Grant is a great weapon for STR faith users who want karmic justice also in the fight).
---This means the weapon that would add the most to the damage is Moonlight Greatsword, featuring an S scaling on INT and +280 scaled mag dmg with 47 INT (my personal, original build). The dmg output is too much less than crystal catalyst (287 mag adj) 937 vs 1116 dmg (with dusk crown + firstborn ring) against forest treants for you to really think 7 mag adj made that gap of dmg. My point here is: don't expect scaled-on mag dmg to be exactly the same as mag adj.
I have tried for a while to make a damage formula to no avail with the time I spared for it, but I hope this helps you with understanding Karmic Justice and utilizing it to its fullest.
(btw if you were wondering what the most dmg I did with it, it's about 1607 against a player in light clothing (with red tear in effect) that I fought last night) Have yet to be buffed up with trident or dragon body.