Guild Wars 2

As stated, both are important. Take toughness if you have good condition removal in your build (signet of purity etc) as vitality only really offers an advantage against dots and conditions. Toughness is great for mitigating spikes and it synergizes better with our less than stellar healing. Remember that one of our trait line skills in CD offers a 5% conversion of precision to vitality, so utilize that especially of you are stacking damage and crit chance. The acrobatics line also buffs vitality and is a common go to trait line for most thief builds.

Consider that it’s hard for thieves to get any toughness save from gear/speccing into shadow arts trait line and it makes sense to invest in some toughness gear/knight’s armor.
Vitality is important, but remember that if you’re stacking precision, which most thieves do anyways, the 10-point trait practiced tolerance can give you an extra 1k+ health.

The toughness gained from a full set of Knight’s armor goes a long way and is definitely noticeable in game. It is usually the difference between being 1-hit by many boss fights/mobs (thinking graveling stalkers in AC) and evading out alive.

You’re more useful alive than dead in a party or WvW, even if you’re not hitting those giant crit numbers.