Finally tried Vaal Earthquake and it does decent amount of damage, can even kill a boss entirely.
Vaal EQ does 7 aftershocks as you walk, during either 12 seconds with no duration effects,
8 seconds with just the 30% less helm enchant and 3.6 seconds with less duration gem. However it does not synergize well with the Less Duration in general. You get much less “normal” aftershocks (with manual swings), as well as your Vaal EQ aftershocks happen very fast and then you have a huge delay before you can start gaining more souls.
So in the end it’s a tradeoff between smoother “manual” damage or between a shorter burst of “automated” damage.
The only decent situation where Vaal EQ can be really useful is to use
https://pathofexile.gamepedia.com/Soul_Catcher
https://pathofexile.gamepedia.com/Soul_Ripper
Soul Ripper is 6 exalts now in Incursion. If it becomes cheaper… maybe it will be good.