It looks like instead of using the 'Use selected bones for flexi-binding', you bound the entire layer to the red bone in your screenshot.
If an entire layer/group is bound to a single bone, then you can't bind specific points.
To fix it, select the vector layer and go to Bone -> Reset all bone rigging
That will free the layer from that single bone. Now, to link the vector layer to the vitruvian bones' chain, select one or more bones from it and go to Bone -> Use selected bones for flexi-binding'.