This was first meant to be a complaint about how Overguard prevents this ability from working in squads, however DE said in a hotfix there will be some changes to Vex Armor so he can build up the bonuses without relying on damage (Melee/ability kills for armor buff, Ranged kills for damage buff).
The next problem I see is this mod has difficulties sustaining itself once you get into the higher level content. And on larger maps like the Zariman and Deimos Labratories it can be difficult to stay around squad mates so their kills can extend the Duration too.
My suggestion would be to increase the Duration added per kill to 2 seconds(possibly let it be affected by duration). Then to have the augment increase the max duration of the ability to 60 seconds.
I would also suggest to revert the 5% healing to just that, instead of over 3 seconds, because that leads to very little healing unless you increase Chroma's health substantially. Thats all I got.