Put the raspberries in a blender or food processor and puree. Using a rubber spatula press the raspberry puree through a fine mesh sieve into a medium bowl. In a large pitcher combine the frozen concentrated lemonade, strained raspberry puree, the vodka, 3 cups of cold water and the seltzer. Stir and pour into glasses filled with ice. Garnish each glass with a sprig of mint and a lemon slice.