I am using Blender 2.78. I enabled "Display modifier" but it is still not visible in editmode.
This is by design, you never see the verticles of the mirrored part, as that would be highly confusing. Those verticles (the mesh) doesnt exist, so you dont see the editable parts of the mesh but you can see the mirror aplied to it. You can only alter the left side of your circle and thus you see that in mesh edit mode.
it is though possible after you aplied the mirror to mirror X editing, and edit both sides of your mirror.