So, I opened the original out-of-the-box "viwi174_release_v1_1.blend", to which I appended the harness then to. Then assigned the harness to use the Viwi's original Armature, not the newely post-append Armature.001. Exported them as FBX and unchecked "Add Leaf Bones" just to be sure. Imported it to the unity package with the Viwi in it, dragged it to the scene and give the left leg a pose to see if the rigging holds and pressed Play:
so basically in unity you have to pair the armature of your clothing to the base avatar armature. There in lies the difference as resonite does that part for you (I assume) but for vr chat you do it yourself either with vrcfury a plugin that does the manual stuff automatically or you can parent it manually by dragging all the bones into the hierarchy. This is where the problem is as there seems to be extra influence where it’s not needed but i cannot find where in the blender file