I don't know for certain, but the way I understand it, vulpine items apply after all damage reduction. So if you're attacked and lose 100 hit points, and have 37% vulpine, you gain 37 mana. If you have an energy shield, you take less damage, and the vulpine item makes up for some of the mana loss from damage. It essentially just makes the energy shield last longer. The damage that isn't absorbed by the energy shield will still damage you just as much, though.