My guess is that increasing Vigor will raise our HP

Works 1to1 like DS3

What is this? vigor doesn't affect hp, that's Vitality.

I started with Vagabond at Vigor 15
Every level in Vigor adds 27 points of HP up until level 17, then it adds 28 points.
From level 21 to 22, it adds 29 points.

Unlike ds3, the amount of vigor you get doesn't seem to decline if you keep on increasing it. I have 31 vigor now, and for every additional point, i get 42 more health.

Current vigor 31. Health increase at this point is 42 per point of vigor.
Unlike any other soulsborne game, health increase doesn't seem to diminish if you invest in too much vigor.
Does anyone know how it works in elden ring?

soft cap is 40

40 to 41 gives 26 hp, before that you get 40-45 hp each leveling

40 is soft cap for vigor. From 40 to 41 gives 26 hp. Before that you get 40-45 hp for each stat

Thank you to Luca, who posted how much health you gain per vigor.
Now my question is solved.