In a medium saucepan, combine the water, hibiscus flowers and sugar over medium-high heat. Bring the mixture to a boil, stirring occasionally, until the sugar has dissolved. Allow the mixture to cool for 25 minutes. Add the lemon juice, lime juice, and vodka. Place the ice in a tall jug. Strain the hibiscus tea into the jug and discard the flowers. Refrigerate until ready to use.