So the 2-piece set says: Each stack of Scent of Blood consumed reduces the cooldown of Vampiric Blood by 5 sec. So lets be conservative here, lets be really conservative and say you'll have a 30% uptime on Vamp Blood. Chugging through those scent stacks its going to knock that CD down after all.
Vamp Blood can be either a constant healing boost if glyphed, or a boost in HP and heals while unglyphed. Quite a nice gain, its stronger healing DS's and more inut from healers.
Now, for the 4-piece: While Vampiric Blood is active, your Blood Shield cannot be reduced below 3% of your maximum