Question about assigning vertices to bones

Hi, go to edit mode & select all the vertices that you want to assign to the head bone (you can press l to select all linked vertices or select it manually).
Go to the right panel & on the Vertex Groups panel select the bone that you want to assign, in this case b__Head__.
Make sure the weight is 1 & press "Assign".
On another note, if you don't want the helmet to morph with slider you can vertex paint the helmet with black.
Also if you want to hide the head underneath you can edit the ExcludePartFlag value HERE or you can use a reference that already have the head hidden like the robot head mask.