Guild Wars 2

Level 80 Thief, 79% Crit-Chance
Sundering Strikes: 40% Chance to cause vulnerability for 5 seconds on crit. (1sec ICD)
Superior Sigil of Frailty: 30% Chance to vuln on crit. (2sec ICD?)
Skale Venom: ??% Chance to vuln on crit. (?sec ICD)

Spamming Unload and Vital shot I’m barely reaching 4 stacks of vulnerability and mostly sticking at 1-2.

Am I doing something wrong?

I’m not an expert on thief, so I don’t know if there are other, better ways to stack up vulnerability, but condition duration is a big part of being able to stack conditions.

Look into runes that extend condition duration, especially if there are any that apply to vulnerability (can’t recall off hand) and consider using “Giver” weapons (+10% condition duration each) and/or using condition duration foods.

Also keep in mind though that no matter what, those ICDs really hamper the ability to apply vuln quickly.

Yep can tell your condition duration is low, also without specific skills to raise vulnerability its hard to gain stacks. Both traits and runes fail at it and are used mostly for keeping stacks from refreshing.

I.E. a warrior pops “on my target” for 10 stacks of vuln, can now auto attack keeping it around 10 stacks until the skill refreshes. You will never get 25 stacks with those items/traits alone.

Every hit has a 31.6% AND 23.7% chance of applying vulnerability, best case scenario you get 2 stacks per hit, worst case you get 0.

For a single unload, you will on average get 2 hits of vulnerability, for vital shot you will get 1 vulnerability every 4-5 hits on average. Given that Vital shot has a cast time of 0.5 seconds + 0.25 seconds after-cast delay and travel time, you will get at best 3 vulnerability stacks before they start wearing off.
Given that unload has a cast time of 1.75 seconds and 0.25 seconds of after-cast delay and travel time, you will get at best 5 stacks of vulnerability before they wear off.

End result, you can get an average maximum of 8 vulnerability by using that trait and sigil, which is ultimately not worth using over other possibilities.