Toughness and vitality are both useless depending on which class you're playing. There's a video of an elementalist soloing AC p1 with full berserker gear.
I find that this game puts survivability more on the player's skill rather than gear. Even with the best tough+vit gear you don't really want to get hit by, say, Alpha's AoE hits in CoE.
Time your dodge, move around more, look at the mob animation. Go toughness+vit if you suck, go berserker if you don't suck. In higher levels fractals almost anything could be fatal if you don't dodge it or use things like smoke screen or feedback, even with the best toug+vit gear, so...
It's different in PVP though. Bunker builds really shine in PVP, where they absolutely don't in PVE, where you could just go full dps with CC and a few survival utility skills, because in the end, you really can't "tank" anything, this game is more about damage avoidance with timing dodges and using the right skills (distortion for a mesmer, blocks/invincibility skill for a warrior..)
For big crowds of mob, you want someone in the group that can cripple them a lot, and another who does huge AoE damage.
So far I've been very successful with guildies who are full berserker, all of them. And I had shit runs with pugs who couldn't even dodge kholer's pull.
During one fractal run with pugs I soloed most of the grawl fractal boss (they kept dying) with a thief in full berserker gear, but signet of malice for health regen on hit, plus traits that give me a lot of intiiative and a few cond removal. My only non berserker gear was a set of rune of the monk, giving me and the group aegis every time I used my elite (and I used basilisk because it had low cooldown), plus bonus on healing. Things like aegis and heals are far more useful than vit and toughness. Also, look up combo fields. Having an ele or ranger in the party, and spamming 2 on the shortbow can heal the whole group from 20% hp to 100% hp.
IMHO anyone who runs with a full set of cleric-like gear is like dead weight in the group, not killing the mobs fast enough and too cowardly to rely on his own ability to avoid damage, to CC the mobs and so on.
Even during parts like CoF p3 and the dredge fractal doors you have to open standing on a platform while mob respawn constantly you don't really have to tank, just get some friggin'
ash kits for fuck sake.
