One of my characters is a Savant with enough points in Vitality to get boosted stats with Oracle-oriented gear, and I have the materials to raise his Regeneration stat to a level pretty close to the absolute maximum Regeneration possible.
How useful would a build like that be in PvP?
Not viable. Regen is basically useless because of how bad it scales
I ran full regen Knight before the vitality update, and even back then it was pretty bad at everything except not needing to wait as long when stalling out a strong NPC from a safe zone.
I don’t really use that slot anymore because regen got nerfed and these days it’s not even really enough to make up for vitality self-harm anymore (also the arua got replaced by an imbue that requires sacrificing a valuable weapon slot)
Vitality builds can be good but u got to build it right to be honest