The reason Vial has been so overpowered is the fact it's burst damage. This fix will affect the burst component, making it 10sec ICD, but only 33% of the original damage. First of all even in PvE this will be a dps nerf for most classes that use this trinket, let's take hunters for example:
The proc chance is 15% and procs of all ranged attacks, so you'll need an average of 6 shots to make it proc, as MM this means an average of about 5-6seconds. If the ICD is 30sec, this results in an actual average CD of 36seconds. Now if they change it to 10sec ICD (15% proc chance), the ICD will be 16seconds, so the average time to get 3 procs will be 48seconds, which is way higher than 36seconds --> Heavy dps loss.
Since Vial's already behind on Wrath of Unchaining, this pushes it even further down the ladder.
Now back to PvP, if Blizzard removes the burst component and spreads out the dmg more, it's probably way better to take a Wrath of Unchaining if you want to keep high pressure overall or an on use PvP trinket for controlled burst.