Question about Void Manipulation

Testing Blade Vortex atm. Most Guides recommend Void Manipulation, which I dont understand.
Void Manipulation increases Chaos Damage - but Blade Vortex has no Chaos damage. Zero + 20% more Damage = still Zero.
What am I missing?

Thank You

It also applies to the poison you inflict.

Hmm, any idea to what extend it helps with poison damage?
And does "also" mean there is some chaos bonus somewhere when combined with BV?

First and foremost, Void Manipulation clearly states that it "supports any skill that deals damage". It does not care if the skill in question also deal elemental and/or chaos damage. You can indeed support a Blade Vortex with Brutality and Void Manipulation - the net result will be Void Manipulation incresing the mana cost of BV and giving you nothing in return as you will only deal physical damage because of Brutality.

Poison damage is Chaos Damage over Time. Its base damage is a portion, usually, of the Physical and Chaos damage of the skill that delivers poison. Thus, Void Manipulation in a BV that does not convert its physical damage to chaos will just result in a stronger Poison and a weaker elemental damage with hits (if physical damage is converted to elemental for some reason) and ignites (if among the elements there is Fire).

Notice that, unlike elemental ailments, Poison and Bleeding are not inherent of physical and chaos damage. In order for a skill to deliver poison, it must have a chance to poison, either from a support (ex. Lesser Poison Support), equipment ("+x% Chance to Poison") or Skill Tree. If your BV build has 0% chance to poison with a BV, then Void Manipulation does not help you at all, and will only raise the mana cost.

Thank You Maxtrux