Your attack damage cannot be reduced below zero. Even if your attack damage is reduced to zero or you use an attack that does no damage (such as Distracting Blow), you will steal the full amount of life stated.
For skills like Barrage and Dual Shot, which hit with multiple attacks simultaneously, each arrow counts as one attack; thus, only three arrows from a Barrage will be affected.
Note that you must hit the target for this effect to work. A damage counter must appear for the hit, even if the damage counter reads a yellow "0" above the target.
Consider combining this with Triple Shot, as one would inflict a massive armor-ignoring spike combined with a decent heal.