There is a gem that increases lifesteal but it seems that since the weapon the does not innately have lifesteal the value of lifesteal is 0 and therefore the percent increase amounts to nothing.
Has anyone found a weapon with lifesteal affix?
I just slotted the “lifesteal increased by x” to a blank 2h sword (blister, unenchanted) and it added lifesteal to it. the gem is actually giving lifesteal, as described on the enchants google doc.
in my setup it gives me 3 hp per hit.
“Increased Lifesteal - GIVES you lifesteal. It doesn’t increase other sources of lifesteal. Healing increases scales lifesteal.”
but you know that already 
I socketed a a lifesteal gem into several weapons and theyve all worked, offhands too, sounds like you may have a bug?
This was posted at the Breach launch, lifesteal has significantly changed since then.
Ignore.
Post-TheBreach Patchnotes note a single change: