This is a great exercise in using your V tool!
Our centre vein (and the fern as a whole) is a 'sunken' carving: sunken below the flat surrounding wood, which we need to protect.
Tilt the V tool as you cut so that the side against the surrounding wood is vertical, away from the centre vein itself. You may have to reverse direction to keep this edge clean. Make sure your tool is sharp, run a light cut first and then firm up.