Vitality Formula:
Total vitality = (Applied Vitality Bonus)*(Constitution + "From Gear" )
Applied Vitality Bonus = 1 + "Vitality Bonus"
Vitality Bonus = "From Boost" + "Lone Wolf" (+ maybe other)
Applied Vitality Bonus is the result of applying the "Vitality Bonus" to the character's Vitality.
For example, if "Vitality Bonus" = +45%, you add your Vitality with 45% of that Vitality and get 1+45% = 1.45, which is the Applied Vitality Bonus.
You can thing of the second sum, that is, (Constitution + "From Gear" ) as your Vitality and ADD a "Vitality Bonus" (as a percentage of that Vitality) to it.
I only calculated Vitality for some character variants, so if you add more talents you may get other terms in the formula and maybe end up with something like this:
Total vitality = (Applied Vitality Bonus)*(Constitution + "From Gear"+ maybe other terms )
The Constitution grows with the player Level.
Vitality Info Box:
At a Hard gamed difficulty, for a character with "The Lone Wolf" Talent, when hovering the mouse over Vitality, you get an Info Box that says:
Vitality Bonus: +45%
From Boost: -25%
From Lone Wolf: +70%
What this means:
Vitality Bonus is written in white and is a SUMMARY of the total "From ..." bonuses. So Vitality Bonus is given by the contribution (Sum) of "From Boost" and "From Lone Wolf", (etc.).
"From Boost" is written in red and is given by the Game difficulty level which can be Hard, Medium or Easy in Divinity: Original sin Classic.
"From Lone Wolf" is written in blue and is given by the namesake Talent.
Examples:
Example 1:
at Hard difficulty, the Character has:
(Constitution points = 4), Level = 4, Total Vitality = 189, Constitution = +131, Vitality Bonus = +45%, From Boost = -25%, From Lone Wolf = +70%
Vitality Bonus = 45% so you add your Vitality with 45% of that Vitality and get 1+45% = 1.45
Total Vitality = 1.45*( 131) = 189.95 which is rounded by the game formula to 189
Example 2 - gear has HP bonus:
at Hard difficulty, the Character has:
(Constitution points = 4), Level = 4, Total Vitality = 223, Constitution = +131, Vitality Bonus = +45%, From Boost = -25%, From Lone Wolf = +70%, From Gear = +23
Vitality Bonus = 45% so you add your Vitality with 45% of that Vitality and get 1+45% = 1.45
Total Vitality = 1.45*( 131 + 23) = 1.45*154 = 223.3 which is rounded by the game formula to 223
This page mentions the following about "Constitution" in Divinity: Original Sin Enhanced Edition
https://divinity.fandom.com/wiki/Attributes_(Divinity:_Original_Sin_Enhanced_Edition)
"
Constitution
1 Maximum Action Points per point
6.2 Vitality per point and level (+7.13 in normal difficulty due to the +15% bonus)
"