its pretty easy explained
IB is a single target spell that has a chance to apply vul, its not guranteed.
second the AI is terrible when facing multiple enemies, it tries to chase with a slow movement speed.
Conceited isnt additive its multiplicative too. but yes the vul multiplier is good, if your vul coverage is good. Which it isnt with ib alone, except for bosses and very low density. Thats why i said it benefits other spells more that have a better vul coverage.
True in PvP this is fine, if its a 1v1 situation. but even here it has its flaws, a very fast char can make it that you wont get hit as often, which might translate to a very poor vul uptime, probably why they buffed duration. The new LS with vul serves you much better, but even this suffers from high density if ur not teching into more LS. Which made me think Adam Jackson is trolling, since he only provides this half baked solutions.
Instead of longer duration they should have changed IB to hit in a small aoe.