Alright, so although I've got this from an older version of FCR2, I imagine it's probably the same for 1.2; Vitality regen in combat (without potions) is 0.0, outside of combat 0.33 (which is 1/3rd of what it was in vanilla). The vitality circle of power does not give an absolute boost to vitality regeneration but instead applies a 1.5 multiplier (i.e. you get 50% more regen).
So the circles seem pretty pointless unless you're already using potions.