Vaal Infuser is a currency item used to increase the quality of certain gear above the normal 20% limit, up to a maximum of 30%.
It can be used on:
When you use a Vaal Infuser, there is always a chance the item becomes corrupted. Corruption prevents further crafting on that item, so the decision to use it matters.
Most players treat Vaal Infuser as a late-step upgrade, used only after they are satisfied with the item's mods.

In general, quality improves the base effectiveness of an item:
Going from 20% to 30% quality is a small but real increase, especially on high-end bases. That extra value matters more for finished gear than for temporary leveling items.
Because corruption is possible, most players assume the item is “final” before using a Vaal Infuser.
Using a Vaal Infuser is straightforward:
Important usage details:
Most players apply it after all other crafting steps are done, since corruption can block future changes.
Most players use Vaal Infuser when:
In general, it is not used on early or mid-game gear. The risk is usually not worth it unless the item is already strong.
Vaal Infuser does not drop everywhere. It has restrictions on how it can be obtained.
The most common way to get it is through Atziri's Temple:
Most players encounter Vaal Infuser by fully upgrading the Smithy room and choosing it as a reward, rather than crafting directly.
In general, players choose Vaal Infuser when:
The crafting bench is safer and more flexible, but it cannot push quality past the normal limit. Vaal Infuser trades safety for extra power.
Vaal Infuser is a specialized currency item used to push weapons and armour beyond the normal quality cap. It is most useful on finished gear, where the extra quality provides meaningful value and corruption is an acceptable risk. Most players encounter it through Atziri's Temple and use it carefully, knowing it is usually the final step in an item's progression.