As a DRK, if you're talking from a survival perspective, Strength has two roles for you:
  1. Living Dead: Prioritizing Strength allows your healers to recover you from Living Dead more easily. Every point in Vitality will make it so they have to heal you that much more to get the buff to fall off. Of course some may argue that you should almost never have to use Living Dead if you're doing your job correctly, which would make the point moot, but Vitality actually works against it.
  2. Souleater and Abyssal Drain: Both of these attacks heal you relative to how much damage you do, so maximizing your ability to do damage with them will help you self-heal for more.

Vitality has two functions as well, but they're generic to every tank:
  1. More HP: Inherently, having more HP will mean you can take more damage before you die, plain and simple. But it in no way allows you to take less damage--it just means the WHM may be able to get another Stone II out before having to heal you. This will also be more accommodating to newer/undergeared healers, who might not have the throughput to keep up (if you net 16,000 HP loss over an encounter because the healer had to get some coffee, having only 15,000 means you're dead, while having 17,000 leaves you alive).
  2. In-Combat Auto-Regen: While you're in combat, you regenerate 1% of your max HP every three seconds. In endgame, 210 gear, this can mean the difference between a 150 HP tick and a 200 HP tick. In general, this is pretty minuscule, but it is an effect of Vitality. If they wanted to make this more substantial, they'd need to raise it to 5% at least.