Combine raspberries, sliced lemon (if using), vanilla bean, and sugar in a double boiler or heatproof bowl set over a pan of simmering water. Cook, stirring, until raspberries have released most of their juices, about 12 minutes.
Strain through a fine sieve, pressing gently with the back of a spoon; discard solids. Let cool. Pour 1/4 cup syrup into each of 3 ice-filled glasses. Top each with 1 cup seltzer. Serve with lemon wedges.