[edit] @XandeRoot not all other, just after -%X Resistance, not after all flat RR’s because X Reduced Target’s Resistances (Elemental Storm) is applied last (3rd) and Viper is applied in the middle (2nd), after -X% which go first.
Viper’s effect will be pretty small because for tougher monster u usually are close to 0.
You can check out the chart I made in the thread below and its analysis:
I concluded there that if Viper works, it will be rather relative 5% damage boost but often just 0% boost. Sometimes maybe 10% relative boost (rather on trash and with very good RR or if boss isn’t resistant to your damage type at all?). Anyway much weaker than other RR’s. But I don’t remember it very well. On a case by case basis in can be checked in my calculator that can compare two sets of resistance debuffs.
In the sample screenshot above getting Viper gives us 2.8% relative damage gain which is less than 100% Damage in most case.