Step 1Pour the vermouth into a pitcher. Add the tea and set aside for 1 1/2 hours, stirring occasionally.
Step 2Strain the infused vermouth using a fine-mesh strainer lined with cheesecloth and pour the vermouth back into the bottle. Refrigerate until ready to use. The vermouth will keep, refrigerated, for one month.
Step 1In a sturdy medium bowl, muddle together the sugar cubes, lemon and grapefruit juices until the sugar is dissolved. Add ice to chill, as well as the vermouth and gin, stirring well to combine. Stir in the soda and Champagne until well-combined, then strain into a chilled punch bowl.
Step 2Garnish with grapefruit slices and serve immediately.