Toughness useful in dungeons?
I’d say it’s more helpful in the harder dungeons and some fractals than other places. It also depends on the quality if group you’re in and how well you are at avoiding damage too. It takes a lot of toughness for it to be worthwhile and I wouldn’t bother with it at all until you’re able to get good gear i.e. 60+
Having high Toughness reduces direct damage to you by quite a bit. It does nothing for condition damage, although damaging conditions are rarely an issue in PvE anyway.
Best case scenario IMO would be to get roughly equal amounts of Toughness and Vitality, but both stats are still very useful even if you only stack one.
Also consider that Chaotic Transference and Runes of the Undead converts some of your Toughness to condition damage. It’s not a huge amount, but still worthwhile.
i roll a 2k toughness mes for all dungeons including fracts and it works/always worked great. No advantages going with Vit for you illusions as well.