Plain '+Vulnerable Damage%' affixes (not the rarer 'Multiplier'-worded version) share the same crowded Additive Damage Multiplier bucket as every other '[+]%' stat on your sheet - late in gearing, a new one is worth only a small fraction of its printed number, exactly like any other Additive roll.
This specific trap is worth calling out separately from the general mult_crowded_additive_diminishing_returns entry because Vulnerable Damage in particular LOOKS like a premium, build-defining stat (it's core to most meta builds), which makes players over-value additional plain-worded copies of it late in gearing when a Global Multiplier or a 'Multiplier'-worded version of the same stat would be worth far more.
Once your bottom-line Additive Damage total is large, deprioritize further plain '+Vulnerable Damage%' rolls in favor of any genuinely separate x% Global Multiplier, including a 'Vulnerable Damage Multiplier'-worded unique if available.
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