spell vamp is essentially lifesteal, except for abilities instead of autoattacks
Also, an Admin should probably remove the dodge section now

And with 200 Armor, you get 66% damage reduction.

Notice the connection here; the values are the same in either direction.

More in detail.
What is actually happening.
Armor/(Armor+100).
This is how you calculate the reduction above 0.
Below 0, the calculation is 2 - (100/(100-Armor))
This is also true for MR

Now, it's accurate to say that each point of armor you have is a 1% increase to your total lifespan against that type of damage (when you have 100 armor, you have a 50% damage reduction, because it's "doubling" your life. At 200 armor, it's a 66% reduction, or 300% life, etc).
I'm not sure the correct turn of phrase for a similar effect for negatives (as saying "it counts as a 1% decrease to your life" doesn't really make sense past 100 shred)

Epic information! Thank you very much for taking the time to explain in detail. This is an absolutely amazing breakdown for new players like myself.

So nice, so helpful.