You can remove Power Drift to run this build without Umbral Forma (3 Forma Build).

For Elemental Ward I recommend running Fire since it increases your health which works well with armor bonus from Vex Armor.

No need for Armor mods or Arcane Guardian, max strength with Vex Armor already provides 90% damage reduction (which is armor capped) so Arcane [Guardian] does nothing for you just like any other additional armor.

Regarding Adaptation I would expect it to have diminishing returns due to how high the armor is.

In the first slot, Streamline vs Fleeting Expertise vs Constitution vs Augur Message. After comparing the overall duration vs efficiency on Vex Armor, I can safely say that Streamline provided the best energy/duration ratio on this build.