-40% healing basically just delays her powerspike by another item. Instead of cleaver tiamat, you have cleaver, hydra, vamp scepter and suddenly you're just peachy. Bramble Vest is only really effective on tanks because fighters NEED damage in order to trade. But Tanks still get screwed due to their immobility, having their engage CC parried, or her % max health true damage negating the armor that came with bramble vest.
Unlike building zhonya's to counter Zed, vest offers no offensive capabilities. you could argue calling, but then that would still gimp you 800 gold and you'd either end up building mortal reminder, or just selling i later on. In either case, by virtue of her kit, she can build glass cannon on first two items and just drain tank during duels, while her opponents are forced into buying grievous wounds not to win lane, but simply to have a chance to kill her with jungler assistance.
Vest becomes even more useless when she finishes bloodthirster, gaining the grieveoused equivalent of an ungrievoused Ravenous, and then all of that healing is further amplified when she inevitably buys spirit Visage.