If you notice Vex/Attune help your Luopon then adding MEVA will only help further.
Luopons aren't effected by Geomancy, even their own aura, like any pets. FWIW, I don't think it's worth going for any DEF, EVA, MEVA, etc. Nothing to really back it up, but when you're sitting at -87.5% DT and you're still taking hits for 300-500, I leaned towards thinking Luopon base stats are terribly low to the point that you're bottomed out on any damage calculations.
With Merlinic and Almaric being your go to for MAcc and MAB, I'd just make a set with regen and DT in mind and another set for Enhancing duration, or a single Regen + Enhancing duration set if you've got Idris, or if there are other options to cap -DT now.