Combine 2 cups fresh cranberries, sugar, and water in a saucepan. Cook over medium heat, stirring occasionally, 8 minutes or until cranberries begin to pop. Pour mixture through a wire-mesh strainer into a large pitcher, discarding solids; cool completely (about 1 hour). Stir in chilled club soda, fresh Key lime juice, and, if desired, vodka. Serve over ice. Garnish with Key lime slices and fresh cranberries.