One of the simplest ways of doing this would be to make it behave in the way that you (ZoS) want it to. Namely, you want it to be a disengage and not a tool used to cheese mechanics in a trial (even though the mechanics are the actual problem, but that's not pertinent to this discussion), which is fair. it's your game and if you don't want people cheesing content, i can learn to live with it. However this kneejerk reaction where you just outright remove the ability from PvE is extremely rash and will only push people to just give up on vampirism altogether.
This skill line already suffers from being so disjointed, it really doesn't need you to kill one of the only decent skills it had and leave it in a much worse spot than it was to begin with. If you want it to be a disengage as you so claim, then change the skill to fit that and not simply remove it from PvE. Doing so shows that your reasoning for changing it is just a facade used as justification for gutting a skill that lets players skip mechanics in a trial and you couldn't be bothered to fix the trial's mechanics.
So, with that in mind why not rework the ability into an actual disengage? The ability itself is very niche and by making it into an escape ability you would remove the 'cheese' aspect it brings now without making it completely unusable. For instance, why not make it into a variant of Streak?
Mist Form: Transform into a cloud of mist, surging forward in the direction you are currently moving, any enemies you pass through take (X) magic damage and you take 20% less damage for 5 seconds.
Skill morphs:
Lingering Mists: Transform into a cloud of mist, surging forward in the direction you are currently moving, any enemies you pass through take (X) magic damage and you take 20% less damage for 10 seconds.
Bat Swarm: Transform into a cloud of bats and surge forward in the direction you are currently moving dealing (X) magic damage to any enemy you pass through and you gain health equal to the damage dealt.
This way the ability retains its use in all content, has an ability that provides a decent damage mitigation and escape, and can no longer be used to cheese mechanics where you have to soak a lot of unavoidable damage.