For the syrup: Put the sugar, water, hibiscus flowers, and ginger in a medium saucepan. Bring to a boil over high heat, stirring occasionally. Remove from the heat and set aside to steep for 30 minutes. Strain and refrigerate until chilled.
To make the cocktail: Combine the tequila, 2 cups of the hibiscus syrup, lime juice, and orange liqueur in a large pitcher. (The vampiro can be made up to this point a day ahead and refrigerated.)
To serve, fill a cocktail shaker halfway with ice, add the some of the vampiro and shake until chilled. Strain and serve straight-up in a cocktail glass garnished with lime slices. Alternatively, serve over ice in a rocks glass.
Hibiscus flowers can be found in most Latin markets and is often labeled by its Jamaican name Sorrel.
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