Whirlwind-style builds keep near-100% Vulnerable uptime because Vulnerable application is baked into the same skill already being spammed for damage (along with Bleed and Slow), rather than requiring a dedicated Vulnerable-setup skill and rotation slot.
This is the general uptime-loop principle behind most 'set and forget' Vulnerable builds: any skill that both deals your primary damage AND applies Vulnerable removes the sequencing problem entirely, since every cast is simultaneously your damage and your setup.
Uptime loops are strongest when the buff-application step has zero opportunity cost - piggybacking Vulnerable onto your spam skill means there is never a choice between 'deal damage' and 'refresh Vulnerable.'
When choosing between two similar skills/Aspects, prefer the one that folds Vulnerable (or your build's other key debuff) into your main damage-dealing action over one that requires a separate cast.
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