Place the mint and sugar in a small saucepan with 1 cup water. Bring to a simmer, stirring until the sugar dissolves, and then remove from the heat and let stand 15 minutes. Strain the mint simple syrup into a large pitcher, and stir in the lemon juice and 6 cups cold water. Serve over ice and garnish with mint leaves and/or lemon slices. Add a shot of vodka to each glass, if desired.