… The % of Attack damage converted to Health, damage from any basic attacks against targets in the sigil or is the damage from the SoC skill AS the attack?
If the latter is this health conversion gained from all sources of damage by the sigil… that is to say at level 12 with 12 points in destruction as well (all things being equal and with no equipment modifiers and 0% resistences of enemies in the sigil) …
Does the player gain 30% of damage from -
94 Vitality Damage = 28.2
84 Chaos Damage = 25.2
65 Fire Damage = 19.5 … for a total of 72.9 health gained from any targets the sigil was cast on?
Yep, this is one of those unclear tooltip descriptions that make you wonder whether the effect is part of the skill itself or is a universal buff to your character (while the skill is in effect).
Unfortunately, the only way to be sure about these things is to test them. Fortunately, SoC is pretty easy to test.
Basically yes, you gain a percentage of the total damage done by the skill (including the Destruction upgrade) as health. It doesn’t really matter that it does multiple types of damage, the lifesteal calculation happens after all the damage reductions/bonuses/resistances are calculated AFAIK.