The thing that should be avoided is for Soul Eater or Scourge to become a superior choice. If Scourge isn't as good a blood gain as soul eater and inferior damage, it might as well not be there. Hence, why I thought of the blood over time effect: it can have superior blood gains, but spamming it is a blood loss, so you just use it to reapply the buff.
There are arguments for why Tanks should never directly boost the damage of other jobs, but I think by this point DRK needs any help it can get. But would the vuln debuff compete with the likes of Contagion, or be it's own class of debuff?