When I was playing Vitalist in a Way of the Wicked run, I was the sole healer. We only ran until 4th before the group fell apart.
However; it IS true that my role, which was demanded of my every round, was to support. I also made it worse by having the Life Leech archetype, so I would try to drag enemies into the collective to drain THEIR hp, to heal allies. (lvl + Wis x 2, save = half. Therefore, infinite healing so long as enemies were near).
It is true that the only was to challenge the vitalist at that point, is to make him switch up his game; combine HP damage with status effects; he can only affect one of those at a time.
Also; there will be moments in Kingmaker where they are not resting after 1 battle, like the bandit fort. Use those moments to harass the party hard.