I am building my Gaurdian to be very tanky but I can't help but wonder. Which stat provides more survivability? Toughness or Vitality? Is it situational?
There are some trade-offs. Overall, I consider Vitality to be a little better, but it will depend on your build, as well as the enemy you're fighting.
It's probably wise to have some mix of these two stats. In general, I would favor Vitality. Of course, the more you have of both, the more tanky you will be.
If you have ways to mitigate damage-over-time conditions, Toughness might be advantageous, since you can avoid much of the downside. Likewise, if you have very poor healing ability, you might want to stack Toughness so you don't have a massive health pool you're unable to refill.
If you have ample heals, especially some form of continuous healing, Vitality will probably be more advantageous, as you can keep that huge health pool refilled.