Third Woe - against obstinate perseverance in sin, as if they wished to provoke divine judgments.
iniquity - guilt, incurring punishment [Maurer].
cords, etc. - cart-rope - Rabbins say, “An evil inclination is at first like a fine hair-string, but the finishing like a cart-rope.” The antithesis is between the slender cords of sophistry, like the spider’s web (Isa 59:5; Job 8:14), with which one sin draws on another, until they at last bind themselves with great guilt as with a cart-rope. They strain every nerve in sin.
vanity - wickedness.
sin - substantive, not a verb: they draw on themselves “sin” and its penalty recklessly.