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.
A tanky guard needs:
Regen - from traits (Absolute Resolution!), food, and Runes (Dolak for example)
Healing - Healing Points!, Mace, traits AH or Monks, dodge heals,
Condition removal or/and Condition duration reduction.
I find 3,400 toughness good enough to take a burst and to survive. You need passive healing and several active heals, plus dodge rolls to heal. Guardians have silly amounts of healing if traited so. Balancing your other play styles to suit is the key.
Personally I rate Toughness over Vitality as it also plays into high crit builds with Retributive Armor. I melt less in sPvP!