Unfortunately, it isn't for a lot of content. I can and have collected seven vuln stacks from the last boss in Grand Cosmos and he [i]still[i] doesn't do half my HP through cooldowns. I've had four in E10N and never come remotely close to dying. Granted, DPS and healers can't collect every vuln stack but this is largely the problem with how low casual content is tuned. People aren't going to respect mechanics when they aren't threatening because there's no reason to. If I can eat Shiva's Axe Kick as a melee, why am I going to disengage and lose damage? It won't kill me, and I can easily heal myself back up with Bloodbath.
The devs need to stop tuning content so low if they want people to care about vuln stacks.