Okay so I thought about this earlier today while driving my car, totally random at that, but anyways, so I was thinking, what is the main difference with lifesteal and spell vamp?
I mean aside the obvious of Spell Vamp only working with abilities, but what I want to know is, is Lifesteal only for basic attacks, or all around damage?
Do they stack? As in a 12% LS and a 20% SV = 12% LS and 32% SV.
I haven't seen this discussed so I thought it'd be some good info to have.
Kazekuro2 votesNo, some abilities have effects that trigger on hit effects, like 
's Q which will proc Lifesteal, but Lifesteal wont proc on his ultimate at all. Only Spellvamp will.
To make this easeir to understand.
Lifesteal Triggers on Auto Attacks and Abilites that proc On-Hit effects.
Spellvamp Triggers on Ablites only.
However, I am uncertain as to whether something like Ez's Q while Knowingly proc'ing Lifesteal, will also proc Spellvamp, because at its heart it is an ability.
THExJACKAL1 votesOkay, so basically LS = On-hit not overall damage. SV = Ability
But as stated, and as you mentioned, if the ability is On-Hit, do they stack? That is probably the more important question in there. Hopefully someone will be able to answer that properly. Maybe even someone from Riot.
Lastly, I just thought of this, does On-Hit exclude Area of Effect abilities?
Also, in the case of 
's W, he turns into a pool of blood dealing damage to enemies that he is directly under. Would that fall under both On-Hit AND ability, triggering LS and SV? Or simply just SV?
I like to get very detailed I know, lol.
Kazekuro1 votesTo my understanding. Spellvamp is a 'direct result equation' unless the Ability in question is an AoE. Which results in 1/3 less. AoE is not classed as an On hit effect. Meaning no Lifesteall will be added to the equation.
Spellvamp Example.
If 25% Spellvamp hits a target for 100 Magic Damage, and assuming the Target has no Magic Resistance. It will yield 25 Health. Or 25% x 100 x 1 = 25
AoE Spellvamp Example.
If 25% AoE Spellvamp hits 4 Targets for 100 Magic Damage, and assuming the Targets have no Magic Resistance. It will yield 33.3 Health. Or 25% x 100 x 4 x 1/3 = 33.3
THExJACKAL1 votesOkay cool. That explains more for me. I appreciate all the info you've given thus far. Thank you for that.
I guess the only thing left unanswered is if LS and SV stack when an ability is an On-Hit effect.
The Veiler1 votesI looked this up the other day because I was curious as well. They do not stack. If you were to get spell vamp on someone who has on-hit effects in their abilities, spell vamp would not apply at all. If it says "applies on hit effects" then just stay away from spell vamp because it does not stack with life steal
ThatGuyNamedAce1 votesOn the other hand, 
can lifesteal on all abilities and basic attacks, ult or not. and gains no benefit from spell vamp.
I'm pretty sure it depends on the type of damage you're doing, physical or magic. 
for example, benefits from both LS and SV because she does both physical and magic damage with her abilities and attacks.
cheekz1 votesBelieve it or not, the answer is no, they do not stack. Usually any ability with on-hit effects (the only type where LS or SV could have the opportunity to stack) are meant to never effect spell vamp or Rylai's slow. This prevents abuse for obvious reasons.