Combine the strawberries and sugar in a bowl and let sit at room temperature for 15 minutes. Mash using a potato masher or a fork into a chunky puree. Add the vanilla bean pod and seeds and the lemon juice. Cover and refrigerate for at least 1 hour or up to 4 hours. Discard the vanilla bean before serving.
Put 4 champagne flutes in the freezer at least 15 minutes before serving. Remove the flutes from the freezer and put a tablespoon or two of the strawberry puree and the liquid into each flute. Fill the glasses to the top with champagne.