Vaal Cyclone

There's a simple reason that mobs in the center don't get hit: where attacks are made is identical to cyclone, which means they start at the edge of your character. Once they're within your edges they can't get hit. Even in the center the monsters still vibrate around a little, which sometimes causes them to stick out just enough to get smacked, but this doesn't occur at nearly the rate for mobs properly on the exterior. This sort of attack area definition is sensible for most attacks and spells, since normally monsters cannot enter the player's space. But since Vaal Cyclone removes this restriction it becomes a problematic oversight.

In short, the best way to get around this issue is to crank out your damage, and I suggest using a Cast on Melee Kill combo'd with an AoE. Running Blood Rage, The Blood Dance, the Vaal Pact keystone, and a 6L Daresso's Defiance (doesn't have to be that base, but this is for maximum charge generation during the cyclone; a Voll's when you're near max crit is pretty good, but then you run the risk of getting much less out of if it when VC fails to crit, which is the same reason I don't use CoC in the following, but insanely more out of it when it does; I've even used a Tabula for a long time), I've had a lot of success with the following combo:

Vaal Cyclone + Faster Attacks + Concentrated Effect + Life Leech + Cast on Melee Kill + Discharge

My build also has a high max frenzy charges, so there tends to be a lot of oomph in the Discharges, which just kills more stuff and feeds into the next discharge via Blood Rage and the Boots (average 1.3 frenzy charges returned per kill; with the Daresso's you get an Endurance charge per kill, as well). The Life Leech gem and Vaal Pact is entirely to deal with elemental reflect. If you specifically avoid that you can drop those two. A 5L version is easily created by either dropping the Faster Attacks or the Life Leech (if you can manage it). Keep in mind that Vaal Pact will disable life regen, so the Blood Rage degen will keep you on the move or hitting a potion of necessity. Replacing The Blood Dance with some other pair of boots (such as Darkray Vectors) is not such a bad idea once you've picked up Vaal Pact and lost the handy regen effect. I still like Blood Dance for the extra damage against low life opponents: the point is to get some melee kills in as frequently as possible, after all.

If you want an option that doesn't risk lagging up party members, try something like Ice Nova instead of Discharge. You won't get nearly as much damage output, but you also don't have to worry about Frenzy charge generation as much (which can be hard in a party, anyway, since you have to score killing blows).

And, as has already been mentioned in the thread, when in parties VC serves as a tactical maneuver that will draw mobs into a concentrated region where everyone can unleash their AoE attacks for extreme effectiveness, and will effectively prevent most of the mobs from attacking.

If you use the skill in appropriate situations (which is hard in dungeon style maps, but easily achieved in more open ones) you can actually end up with 2/3 or more of a charge towards the next use. I've even exited with a full charge on many occasions, ready to nuke another horde right away. The combo devastates large packs of mobs, as even with the Concentrated Effect and no sources of increased area the Discharge hits a wider area than the VC can.

It annihilates the routine map bosses before the 5 second VC duration has expired in all but a few exceptional circumstances (mostly from an unexpected lack of fodder to kill, possibly because they were blocked by a terrain obstruction). Bosses like the Hillock types, that normally go into a second phase (and fully regen their health in Hillock's case in particular) can be dead before the second stage (and health regen) can even trigger. No leap slam zombies to deal with!

I got completely destroyed by Carnage (the Brutus/Kole type Promenade boss) when I used VC near him, so for the heavy-hitting and risky bosses you either need to use non-VC tactics (I'm using a 2h weapon, so I have a well-linked attack that does fine on its own), or plan and test to see if it will work safely. The problem with Kole being that his "harpoon" attack can still be used when he's under the effects of the VC, and so he basically spams that on you continuously. As a side note, this leads to some fairly bizarre effects, as Kole's attack tries to move you but the VC wants to keep your location fixed. I recall seeing myself suddenly appear about half a screen away from the spot I activated VC, then shortly thereafter reappeared there, still doing the cyclone.

Basically, if you would never stand in one spot for 5 seconds against a boss with any other build, using VC is likely to be a bad idea.

The Herald of Storms Nemesis mod is potentially problematic. It has a decent chance to stun you during the activation phase, plus you'll be stuck in one spot for 5 seconds while thunder bolts pelt your head constantly.

If you can encounter Bloodlines, then the packs that spawn bosses carry the obvious risks. If you're going to summon a Beyond boss, for example, you're going to really hope it's right at the end so you can move around, or that it's near the beginning and you've got a bunch of fodder to Discharge it into oblivion. Bloodlines are the one thing I expect to most screw with VC in future versions. Worst case, do as a HC person would: log out.

The biggest problem that can happen for you otherwise is if you suck in an "other allies can't be killed" aura. Make sure to keep your buffing flasks up and always have an instant recovery life flask on hand, and you should be fine. Probably good to try to use your movement skill as soon the VC ends to get out of dodge. Worst case, do as a HC person would: log out.

In any case, mobs rarely make it into the interior space and survive to the end like this, though it isn't impossible. It's never been a problem for me, though. Most of the ones that can survive to the center are high HP mobs that are fatter than you, and so will still be in the attack zone. Melee mobs won't attack due to the constant movement (even in the center they're still vibrating around), leaving just the ranged and spell casting mobs to worry about.

All in all, I find it a very enjoyable skill, and it shouldn't take that long to get a hang of how to use it most effectively and safely. I do hope they decide to make vaal cyclone attack the entire circle, instead of just the annulus exterior to your character, though.

The most super fun thing to do with Vaal Cyclone: suck in one or more tormented spirits and one or more rares/uniques for them to possess, and you can get a massive loot explosion. Extremely amusing in parties, especially with an MF culler (assuming you don't nuke everything yourself; switching to Ice Nova makes that more likely). I like to call it the Loot-nado. I've seen vertical lists of drops so far I had to go up and down from the spot by more than a screen length to see everything.

Also: Beyond maps (either by map device or Otherworldly affix) become super easy to trigger the summons on. I've routinely seen the non-boss spawns slaughtered by the discharges.

Movement speed increases the range the VC can suck monsters in from, so a Quicksilver Flask of Alacrity becomes a DPS/clear-speed flask with VC. Of course it also pulls them in faster, but since you're really just trying to kill a single mob, the faster you can pull them into the discharge range the better.

Yes, a CoC-cyclone is better, but you can actually pull the CoMK-VC off with a much cheaper set of minimum gear (going for a full 6L Voll's/Daresso/whatever is always costly, but a 5L anything still works very well; you can do very well with just basic rares, as the charges come mostly from the Blood Rage skill rather than gear). And it's still a pretty cool effect, especially for the aforementioned fun reasons.