The Scissors tool can be used to cut away lines from an existing vector shape. Select an existing shape and select the Scissors icon in the toolbar or use Layer › Paths › Scissors for the menu bar.
You can then click on a line in your vector shape to cut it away. When you’re finished, click outside the shape or press Enter/Escape to exit the scissors tool. When there is only a single line left, you’ll leave the Scissors tool automatically.