League-of-legends – What happens when Lee Sin uses Resonating Strike on a Vlad using Sanguine Pool

Life Steal and Spell Vamp don't proc based on the damage bonus source of the skill (AD versus AP), nor is it based on the damage type (physical versus magical). It's actually based on the kind of skill.

In general, Spell Vamp is the only drain effect that affects skills. All of Lee Sin's skills, despite all being AD-based physical attacks, use Spell Vamp and Spell Vamp alone. Akali is an example of a somewhat physical champion whose attack skills are all Spell Vamp-based (her innate Life Steal is to benefit her auto-attacks only). Even skills like Ashe's Volley, which applies her Frost Shot effect (but only that, the key point being as you'll read up soon), will use Spell Vamp.

Life Steal generally applies only to your auto-attack, but it also applies to skills that apply on-hit effects. For example, Blitzcrank's Power Fist deals 100% extra damage and pops the target in the air in addition to applying on-hit effects (for items like Black Cleaver). This benefits from Life Steal instead of Spell Vamp. Other examples of skills that benefit from Life Steal would be Irelia's Bladesurge, Ezreal's Mystic Shot, and Gangplank's Parrrley.

As far as I know, there is no skill which benefits from both Spell Vamp and Life Steal. It's always one or the other.

If a champion has less than 0 armor, they receive bonus damage. The damage formula:

2-(100/(100-armor))

If Trundle uses his ult on a champion, he lowers their armor by a percent. This prevents him from dropping an enemy's armor below 0 with his ult alone. With a Black Cleaver or other form of armor reduction, it would be possible to do bonus damage (use the ult THEN get stacks on the cleaver!).

Edit: Armor penetration is applied in this order (source):

1. Percent reduction (Bushwhack / Pounce)
2. Flat reduction (The Black Cleaver, Puncturing Taunt, etc)
3. Flat penetration (The Brutalizer, Sunder Mastery, Runes of Desolation)
4. Percent penetration (Last Whisper)