I stand corrected. I just went out and tested my life stealing items.
Shadowstrike has 100% scaling.
Ancient Hakkari Manslayer has 100% scaling.
Skullflame Shield has 100% scaling.
Demon Forged Breastplate has 40% scaling over 4 sec, 10% scaling per. tick.
Neretzek, The Blood Drinker has 0% scaling.
The weird thing is that all of these procs share the same spell proc, with only the base numbers being different
spell=18817/drain-life (spell=18815/drain-life, this spell procs 18817)
spell=16414/drain-life
spell=24585/drain-life
spell=26693/drain-life
spell=29155/drain-life