Hey all, I found out that both Circles actually do the same thing, namely
Vit. regen +2
Vit. regen in combat +1,
which is interesting because I was expecting the Circle of Vigor (hexagon shape or octagon shape) to actually contribute towards Vigor regen.

Then I tried stacking the 2. So I went to Circle of Vigor first then Circle of Life. What happened then was that instead of the 2 effects stacking up, I got
Vit. regen +2
Vit. regen in combat +1
which is the same as if only one of the Circles' effect was active. Upon expiration of the Circle of Vigor's effect, both Vit. regen bonuses go to 0 despite the Circle of Life's effect still being active.

Can anyone test this to see if it happens to him/her?

If anyone is interested in testing out a) the effects of Circle of Vigor; b) the effects of stacking Circle of Life and Circle of Vigor, I have marked their locations in the Flotsam map attached below.