This ranger trait seems to be broken when you are downscaled. I had the Pet Management window opened while I was killing in Fireheart Rise(downscaled to 67) the Black Bear’s vitality remained at a constant 1,880 as I was killing things. I only went up to 10 stacks. All the other stats were increasing as expected.
Then I tried it in one of the opposing team’s borderland in WvW(for lv80 and a raptor to kill). The vitality increased correctly there after the first kill.