Heat 4 cups of water to boiling. Meanwhile combine the mint, ginger, sugar, vanilla, lime in a medium bowl. Add boing water and let steep for 30 minutes. Strain the mixture of water and herbs into a 2 quart measuring container, pressing on the solids to extract all the liquid. Add lemon juice and enough cold water to make 8 cups. Cover and refrigerate. When ready pour lemonade into glass and sprinkle with remaining mint sprigs.