Grimtools: https://www.grimtools.com/calc/JVljdR7N
Why did I do this?
I wanted to create an Aura build to eventually fun-run through Normal, killing everything including all the bosses without ever attacking or using retaliation.
What happened?
At some point, I realised I’d created something that was both silly and rather resilient. Yes, it can kill all the bosses in Normal without attacking or using retal, so yay, that worked. Also, it can face-tank Lokarr using a pistol and a piddly low-level off-hand Leafmane Trophy, so yay again.
Show me…
Gear
The two main vitality-conversion items are monster infrequents that do not rely on specific RNG prefix/suffix rolls or scaling values. That’s significant, because it means you can use whatever item drops, even those obtained at a low level, without the need for extreme farming. At end-game (for example, Lokarr) I’m still using the Namadea’s Eye I picked up in Normal difficulty. The Leafmane trophy is also low-level, necessary due to the high spirit requirement for higher-level versions.
Namadea’s Eye - drops from Namadea in Barrowholm cellar. Drop rate is about 20% so you may have to kill Namadea a few times before you get one. Note - you have to be hostile with Barrowholm in order to access the area where this boss spawns. I recommend going hostile with Barrowholm in Normal to get the MI item, then switching to friendly in Elite through Ultimate because of the decent vitality items and augments sold by the Barrowholm vendor. Namadea’s Eye has multiple useful damage conversions for an Apostate, affecting both Ranged Expertise (Inquisitor) and Spectral Binding (Necro).
Leafmane Trophy - drops fairly frequently from various Leafmane enemies found mostly in Ugdenbog/Barrowholm areas. Converts 100% fire damage to vitality on Inquisitor Seal. Also adds a flat 110 vitality damage to Inq Seal, and -15% vitality reduction.
Third item for conversion to vitality damage (among other things) is Radaggan’s Mask. You will need to have the blueprint drop to be able to craft this item. Wearing this gives:
+1 to all Necro skills
+1 to all Inquisitor skills
-1 second recharge to Mark of Torment
12% damage absorption to MoT
2 meter target area to Aura of Censure
100% fire damage converted to vitality to AoC
-80 OA to AoC
-80 DA to Aoc
You can see this is a pretty good item for this build!
You can also craft another mask and transmute it to the shoulder armor from Radaggan’s set. Might take a few rolls of the transmute RNG to get the shoulders. If you don’t fancy doing that, pick up some Korvan Pauldrons (they are the ones with Elemental-to-vitality damage conversion) and farm for a decent drop with the resistances you need. Or just wear the best shoulder item you have acquired that suits the build and your specific needs/playstyle.
All the other gear is really just ‘wear whatever you have that best fits the build’. I tend to prioritise resistances over damage in hardcore, but feel free to go damage crazy if you want to… just try not to die 
Relic - Deathchill Aura but only because it’s called ‘Aura’. I know it doesn’t work like an actual Death Aura but, as I said, this is a FUN build. It’s more important that the relic has ‘Aura’ in the name than it actually contributes a massive deal to the build. Also saves you a load of mats not needing to craft a Mythical relic.
Skills
The fairly low number of skills means it’s possible to max them all out.
Necro - all selected skills maxed
Mastery to 32
Reaping Strike
Necrotic Edge
Siphon Souls
Ill Omen
Spectral Binding + Wrath
Call of the Grave
Mark of Torment
Inquisitor - all selected skills maxed except Chilling Rounds
Mastery maxed
Ranged Expertise
Chilling Rounds (soft max - to 10)
Aura of Censure
Inquisitor Seal + Arcane Empowerement
Devotions
I used the devotion path from Nery’s guide for a vitality caster.
Green node
Bat
Viper
Eel
Wendigo
Yellow
Panther
Lotus
Lion
Scales
Remove yellow
Scholar’s Light
Rattosh
Lantern
Remove Lion and Scholar’s Light
Wretch
Blue
Dying God up to the proc
Lokarr
Steps of Torment boss rush
Build highlight reel
Maybe then you could use the Heart of Yugol armor.