Place the lemons, sugar, vanilla bean scrapings and water in a roasting pan and stir. Place roasting pan in the oven and roast, stirring occasionally for about 30 minutes, until the edges of the lemons begin to caramelize. Remove the roasting pan from the oven and cool to room temperature. Place a strainer over the bowl of a food processor and squeeze the juice from the lemons. Strain the liquid from the roasting pan into the processor bowl. De-seed and roughly chop 2 lemon halves and place in the food processor. Process the lemon juice and lemon pieces until smooth. Strain the contents of the food processor into a large pitcher, add 4 to 6 cups water, lemon juice, sugar to taste and ice, stir well. Pour into ice-filled glasses and garnish with mint leaves.