Ok. I've seen some confusion on what stacks with what in regards to Damage Reduction abilities in 3.1.
Here is the lowdown.
(1)BoSanc and Vigilance DO NOT stack.
(2)Renewed Hope, BoSanc, Imp Righteous Fury, Glyph of Divine Plea, and Argent Defender DO INDEED stack. Each DR% is taken off after the other is applied.
(3)Renewed Hope & Vigilance DO INDEED stack. Each DR% is taken off after the other is applied.
Argent Defender's 30% DR takes its percentage off the damage AFTER the DR from buffs are applied.
Below are the numbers as they stack on a Prot Pally. He has 6% passive(spell) DR, so the baseline is lower to start. I used Penance as a baseline because it has only a 1point damage spread.
776 :BoSanc (OR Vigilance)
753 :BoSanc + Renewed Hope
708 :BoSanc + Renewed Hope + Imp Righteous Fury
495 :BoSanc + Renewed Hope + Imp Righteous Fury + Ardent Defender
Once we get the new Glyph of Divine Plea, I will test this as well.
UPDATE: Glyph of Divine Plea has been tested and does stack with all other DR effects.
I hope this clears up some of the confusion I have seen on numerous forums.