Guild Wars 2

How much toughness does a thief need for PvE, dungeons and fractals up to level 20?
How much is too much and causes aggro problems?

No toughness needed. Most enemies you just use Black Powder and perma-blind them. If immune to blind, which is only dredge and champ attacks, use shortbow if you’re uncomfortable meleeing and dodging appropriately.

Putting 3 in Acrobatics is usually all the help you need if you’re new to the class. Gives extra dodge and 150 vitality.

Full berserker is what most thieves choose for PvE. A thief with a lot of toughness will die to a hit anyways, so there is no need for any of it. Just learn what to evade and you’ll be fine.