I deleted the add-on.xml installed by FLAi Operations Center and made a new one an xml editor. That seemed to have worked. vPilot picks up 1884 models though I didn't check if the FLAi effects and textures also work.
The only difference between the add-on.xml provided by FLAi and the one I made is "version 4,4" instead of "version 4,0".
It doesn't seem to be an issue between FLAi and vPilot, but P3D not detecting the add-on.xml properly.
Looking forward to the fix by FLAi.