Place strawberries, raspberries, lemon juice, and sugar in a sauce pan and heat over high heat until boiling. Reduce heat to low boil/high simmer and allow to simmer for 5-8 minutes, until mixture becomes syrupy. Stir occasionally. Berries should be pretty much liquified.
Strain mixture well through fine mesh strainer, pressing syrup out of berries. Allow berry syrup to cool at least 30 minutes or overnight in the fridge.
When ready to serve, pour seltzer water and vodka into a gallon pitcher, add 2-3 cups of ice and stir.
Pour berry syrup into pitcher and stir well. Serve over ice.
Garnish with berries, lemon slices, or mint sprigs. Makes 16 one cup servings including ice.
Non-alcoholic version: Replace vodka with the same amount of additional seltzer or plain water.
Fresh or frozen berries may be used to make berry syrup.