How do you reduce the duration of soul gain prevention?

The wiki states "The length of soul gain prevention time varies between skills, and effects that modify the duration of the skill will also modify the duration of the soul gain prevention time."

However this doesn't seem to be the case with Vaal Arc - it has a base soul gain prevention time of 2 seconds, but allocating 50% reduced skill effect duration (25 reduced from the passive tree, and 25 from Timeclasps) doesn't change the soul gain prevention at all.

I know there are builds that build-out to no soulgain prevention time, my understanding was they did that via reduced skill effect duration - am I wrong?

I could use some help otherwise my league start is bricked before maps :(