Quarter the grapefruits, then your hands or a citrus press to juice the segments into a liquid measuring cup. You should have ½ cup of grapefruit juice. Halve the limes, then juice into the same measuring cup; you should have 1 cup of juice total.
Pour the citrus juice into a pitcher. Add the vodka, sliced grapefruit and lime, if using, and simple syrup. Top with the champagne.
If desired, use the lime wedge to wet the rims of champagne flutes, then dip in sugar to coat. Pour the punch into the flutes. Serve chilled.