Life Steal vs. Spell Vamp

Is there a universal damage formula? Or is it as simple as: <Damage from skill> - <Armor after reduction/penetration>?

It's more like <Damage>*<defense modifier>. Except certain exceptions (Banshee's Veil and spellshield equivalents, Jax's Counterstrike, blinds) and extreme cases (Fizz taking flat damage reduction masteries vs. an opponent that suffers from Tryndamere's Mocking Shout and Trundle's Rabid Bite), you never take 0 damage from something.

If you mouse over your armor and MR stat, they will tell you exactly what reduced damage you take versus the specific damage type.

I believe the basic formula for calculating damage taken goes along the lines of (base damage) / (1 + (defense stat / 100)). So if you have 100 armor, you take only 50% damage vs. physical. If you have negative armor, you take extra physical damage. And if you have -100 armor (lolwut), you die from ANY physical damage. Actually I just remembered that one time I went into a custom bot game as Soraka for funzies and reduced Dragon's MR to -110. But it didn't die instantly xD

On a special note, true damage always does its stated value regardless of resists and damage mitigations. True damage does not do more damage if your opponents have negative defense stats. Depending on the method of delivery, true damage may be blocked by spell shields.