Edit- Wow you helped me again SniperHF. After your post, I realized that there are stats folders inside public/*modname* folders as well. I needed to put the file in the stats folder of that folder. I wasted a lot of time in editing constitution and act part, but now after putting it in the correct folder, I realize that changing only "vitality" does the trick.
And in Public\Shared it doesn't work? I just tried to give a vitality boost to CasualNPC and GEN_Bodyguard in an editor-modded game, and the vitality bonus was applied correctly.
That's something I don't understand. Like I mentioned in post #3, I tested it and it worked until a certain vitality limit(vitality was 428, and it was increased to 648). So, in "hardcorenpc", vitality 80 was same as vitality 100, and ALSO same as vitality 2000.
Later I emptied the shared folder, and checked in-game to find that the npc(initiate guarding silverglen entrance/rogue earth elemetal) had 648 HP.