Hi!
I’ve been following this build up to level 22:
http://www.grimdawn.com/forums/showthread.php?t=29256
However, it seems that it is a bit outdated for B31. Is there any specific way to build an acid/poison warlock? Or are there better ways to make an Occultist/Arcanist combo in B31, like going for Vitality damage?
Any suggestions or builds are appreciated. 
You should be liquefying everything at level 22 with a DEE lock. Could you post a Grimcalc showing your build?
Well I think I followed the build pretty much to the letter and it does OK (energy problems though) but from what I’ve read it’s not that strong in Ultimate anymore, struggling for damage?
I still farm with my DEE Warlock, which is similar to the one you’re following, and it handles Ultimate quite well. It’s gear-dependent to a degree (like many builds) but performs quite well in BoC, SoT etc. and is fun to play.
Energy problems can be overcome with a point or 2 in Mental Alacrity, use of a focusing Prism, and regen from Devotions.
Early i think u can take point in Focused Gaze (make it cd 4s but huge dam), the DEE will 1 shot eveything - you can use Poison bomb as another atk.
And i think this build required some gear to really shine on.
I’ve played a couple of DEE builds and they start to take off around lvl 30-35. DEE feels incredibly weak before then imo.
Several build comments:
Max the warlock ultimate skill, Possession. 10% damage reduction and massive damage bonuses are worth it beyond a doubt.
Don’t put any points into the Solael’s Witchfire line. It’s now geared towards melee, and does not providing relevant bonuses. Also, skip doombolt. You’re not really geared for it, and spamming DEE will be better DPS anyways especially with a devotion proc.
I’d move 11 points from Mirror to Conversion. Conversion is no longer a 1 point wonder (while Mirror still is), and the CC reduction from scaling it will be necessary for kiting on higher difficulties due to all the slows, freezes etc.
Put 1 point in Bloody Pox to proc a devotion skill. Bloody Pox procs out the wazoo.
Build example, with devotion tree (Manticore, Scorpion, Abomination, Chariot, Behemoth for procs): http://grimcalc.com/build/3bEc5H
I link Manticore to DEE, Scorpion to Mark of Dreeg (for point blank aoe) and Bloody Pox to Abomination. The devotion tree also includes enough cunning for Dreegs dagger and lots of regeneration to supplement Blood of Dreeg. Putting all your attribute points into Physique will allow you to use Empowered Legplates of Valor down the line for their op 900 hp bonus. If you aren’t playing HC, I would split physique and spirit and just use offensive gear. I wouldn’t change the skill or devotion tree though.