I think I'm right in saying Vigier don't actually use a truss rod? So they're very confident that the carbon fibre strip will keep the neck stiff.
Thinking about different manufacturers' techniques, on Music Man all-maple necks they slice the fretboard off, insert the truss rod then glue the same piece of wood back on, so you can hardly see the join.
And I think G&L cut the neck in half lengthwise, through the middle of the fretboard, then stick the two back together.
Presumably both techniques affect the subsequent stiffness of the neck, even though both necks start out as single pieces of maple.