In Path of Exile, Vaal skills are granted by Vaal skill gems, a special type of corrupt skill gem that can be used to unleash powerful abilities fueled by the souls of slain enemies.
Vaal skill gems can be looted from corrupted areas, certain strongboxes, and bosses of corrupted maps. It is also possible to create a Vaal skill gem by using a Vaal Orb on a corresponding normal skill gem (e.g. using a Vaal Orb on Spectral Throw has a chance of turning it into Vaal Spectral Throw).
To use a Vaal skill, its gem must first be charged by collecting souls. Players may know, each slain enemy provides one soul, assigned randomly to one of the character’s equipped Vaal skill gems.
Souls will never be assigned to Vaal skill gems in the character’s secondary weapon set. Collected souls cannot be transferred between zone instances; instead, each instance maintains its own soul counter for each gem. In other words, entering a new instance will start a new counter for each gem, but returning to a previously visited instance will restore each counter to its previous value.
Once a Vaal skill gem collects a sufficient number of souls, it is charged with one use. The number of souls per use depends on the skill and increase as the character progresses in the story. Some Vaal skill gems can store enough souls for more than one use. Once a gem’s maximum soul capacity is reached, it can collect no additional souls until they are spent by using the skill.
The story has the following effects on the soul cost of the vaal skill:
Completion of Act 5 increases the cost to 150% of the base value.
Completion of Act 10 increases the cost to 200% of the base value.
Please note that once the cost increase has been applied it will apply to all game areas, including the areas of previous acts.
Because a Gemcutter’s Prism can’t be used on corrupted gems, getting quality on vaal gems is substantially harder than it is on a regular gem. Vaal gems must either drop with quality or a normal gem already with quality must have a Vaal Orb used on it and be turned into a corrupted Vaal gem.
Vaal skills themselves can not be activated by trigger gems, they can trigger other gems however. Any effects that modify the original skill, such as Helmet Enchantments and Threshold jewels, will also affect the Vaal version of the skill as well. Hopefully the above guide will be helpful to you.