When you want to use V-Cuts you should have one single overall board outline and you then add individual V-Cut lines to define the individual boards within that single overall board outline. V-Cut lines can only be straight and must extend from one outer board edge to the opposite edge. They cannot extend only part way across the board and obviously cannot be curved. Just add the V-Cut lines in the Board Outline layer. They do not need any extra  form of definition.