I recommend you to read a paragraph on Resistance reduction from here: Malawiglenn’s guide on game mechanics for beginners - Google Docs
From this you may conclude why Viper’s type of RR is the worst of a kind. Also since Viper gives its RR as a stat it needs Weapon damage skills to work and scales with it up to 100% so it’s even worse / doesn’t do anything if you don’t have it. i.e. Hand of Ultos RR is different, works always in 100% effect because it’s on a proc but still kinda bad being % Reduced Resistances worst kind of RR.
Also you can try using the Resistance calculator to better see it: 🧮 Critical Hit / Resistance Calculators - DPS multipliers
- there’s even a screenshot example there showing Viper only decreasing Resistance by 4% due to how % Reduced Resistances works