Shivermist Buster vs Venom Veiler
Veteran players, tell me from your experience which was better against your fights against Vanaduke; Shivermist Buster in order to freeze him in place or Venom Veiler in order to poison him and deal more damage to him?

As a shivermist user myself, keeping vanaduck from being able to strike back is more important than reduce his damage.

After all these years, Midnight-Dj is probably far from a novice. My post #1 above is not meant as an insult to people who like Shivermist Buster.

Shiv was the preferred bomb over VV when it came to running FSC ~8+ years ago
In terms of whether a Shiv/VV should be used in the current meta or not then all you need to know is that you should instead just bring a Blitz and a Poison Barrior/Vial along with you. While both the VV and Plague Needle are great at inflicting poison it removes 1 Blitz from the fight making the whole thing much slower. People usually bring a Poison Barrier/Vial with them to Vana nowadays since it does the same job when it comes to inflicting poison but you won't be down a Blitz for the fight with Vana.

1) What Bopp and Hatn said.
2) VV can be good for the rest of FSC if you’re in a four person party and you have at least one other person with Electron Vortex. It’s useful for making slags slightly squishier, but it’s pretty redundant in smaller parties. As for the boss fight, 3 Blitz and 1 Plague is ideal for sustaining poison and dealing damage. The clipping into Vana trick renders Shivermist completely obsolete if everyone can do it, and even if you’re not doing that, I’ve found that freeze can make Vana harder to predict.
I agree with Xarbonjinxmesa. Last month i played with a bomb master (god it's hard to master bomb) running VV in 4 party FSC and i still remember it like it was yesterday. It was wonderful but it had a big downside : my game couldn't handle it, even when lowering my graphic quality. I run GTA V and The Witcher 3 at high/very high in 1060p60fps so it's cleary a spiral knights's problem. VV is supposed to help clear faster but it's countered with the freeze when too much ennemies are poisoned ...
In conclusion sadly none of those bombs are that good in FSC but it's always fun to play with a bomb master

It's worth noting that Vanaduke has fairly heavy poison resistance, he's just shy of being immune to Venom Veiler if I recall right.
You're not going to notice much effect from it at all.

Thanks. I ignored the difference in poison strength between Venom Veiler and Plague Needle, but I guess I shouldn't have.

When I started the game way back in 2012, I might have preferred Shivermist Buster (SB) in LagStorm Citadel (LSC). But about a year later, I eventually realized that SB is actually more of a hindrance. Experienced players with adequate gear for farming LSC will prefer to use the following setup:
Sword: Voltedge (preferred) or Combuster (has significantly more knockback than Voltedge)
Gun: Spiral Soaker (preferred) or Blitz Needle Gun
Bomb: Electron Vortex (preferred) or any other vortex bombs
Notice Shivermist Buster (SB) is not on the list.
Vortex bomb is deployed to round up slags into a closely-packed cluster (bigger the cluster, the better) which conveniently allows the players to strike down all targets in the cluster with a charged attack from a Voltedge, dealing tremendous damage. Combuster does deal slightly more damage than Voltedge, but it also packs considerably more knockback from its charged attack, so much so that if any slags are still alive (usually the case in a full party), they are often pushed far outside the vortex radius. It is for this reason that Voltedge is preferred over Combuster (non-issue in a two-person party).
SB, on the other hand, freezes the targets caught in its wide radius, forcing the players to engage frozen targets individually one at a time, thus greatly lengthening the fight. It should be noted that the targets 'frozen' under the effect of SB can still attack (as long as you are within its melee/breath range). SB merely prevents them from moving/changing their orientation for several seconds. Both the duration and range of SB exceed that of any vortex bombs. Thus, if both an SB and a vortex bomb were to be deployed simultaneously, SB will always win, completely nullifying the vortex effect. It is for this reason that veteran players dislike SB in LSC.
For relatively new/inexperienced and/or under-geared players, Shivermist Buster will likely increase their survivability marginally at the cost of greatly increasing the run-time. With improved gear and skill, players will eventually want to use increased mobility and better positioning to mitigate/counteract damage received.
Even against the Vanaduke boss fight (all three stages), it is preferable to go full blitz on the boss rather than freezing the primary target (Vanaduke) and the secondaries (spawns) with an SB. If you are deploying the SB, then you are not dealing damage, thus the fight duration lengthens. Longer the fight, the more you must dodge incoming hostiles.
Dodging the hostiles in a boss fight eventually becomes almost a second nature once the players learn to read the enemies' attack & movement patterns. It is generally recommended that everyone in the party circle the boss in the same direction - this will make it easier for everyone in the party to predict enemy patterns and avoid incoming damage.
Another Note: Spiral Soaker is not just a mere reskin of Blitz Needle Gun. Under the identical UVs, Spiral Soaker will be able to fire the next charged burst sooner than the Blitz. And this difference will be more noticeable when you compare a GM Spiral Soaker (GM = GameMaster = CTR VH + ASI VH UVs) against a GM Blitz Needle Gun. This difference arises as a result of shorter animation on the Spiral Soaker.

SB is nothing but a crutch that actively sabotages some playstyles.
If you can't handle a moving enemy, use Torpor Tantrum instead. At least then vortex and shield pushing still works.
If I notice someone using shivermist, I like to counter it with ash of agni, which negates the freeze without affecting anything else.
Novices tend to like Shivermist Buster, because they can stay alive without knowing how to dodge Vanaduke. But experts, who know how to dodge, would rather just have all party members working offensively. So experts should prefer Venom Veiler over Shivermist Buster.
But if we're really talking about what experts prefer, then the truth is that they'd probably prefer another Blitz Needle or a Plague Needle instead of Venom Veiler. Unless the poison chances are very different from what I expect, Plague Needle seems better than Venom Veiler here, because you get to inflict not just poison but also a lot of damage.