To prepare strawberries, combine the brown sugar and vinegar in a medium bowl, stirring with a whisk; add 2 cups strawberries, stirring gently to coat. Cover and chill 4 hours, stirring occasionally.
To prepare cream, combine the crème fraîche and 1 1/2 teaspoons sugar, stirring well until sugar dissolves. Cover and chill.
To prepare sorbet, combine 2 cups strawberries, 1/4 cup granulated sugar, juice, and pepper in a blender or food processor; process until smooth. Strain the strawberry mixture through a sieve over a large bowl, and discard solids. Pour mixture into the freezer can of an ice-cream freezer; freeze according to the manufacturer's instructions. Spoon sorbet into a freezer-safe container; cover and freeze 1 hour or until firm.
To assemble parfaits, spoon 3 tablespoons sorbet into each of 8 champagne flutes; top with 3 tablespoons strawberries. Spoon 1 1/2 teaspoons cream over each serving. Drizzle remaining juices from the strawberries over each serving. Garnish with mint sprigs, if desired.