* 8 cups water
* 3/4 cup sugar
* 1/2 cup plus 8 teaspoons tamarind concentrate,* divided
* 2 tablespoons plus 4 teaspoons fresh lime juice, divided
* 12 tablespoons vodka, divided
* 4 cups ice cubes, divided
* Lime slices
*Often labeled "concentrate cooking tamarind," tamarind concentrate can be found at Thai and Asian markets.
*Stir 8 cups water, sugar, 1/2 cup tamarind concentrate, and 2 tablespoons lime juice in large pitcher until sugar dissolves. DO AHEAD Cocktail base can be made 1 day ahead. Cover and refrigerate. *Combine 1/2 cup cocktail base, 3 tablespoons vodka, 2 teaspoons tamarind concentrate, and 1 teaspoon lime juice in cocktail shaker. Add 1 cup ice cubes; shake 30 seconds to chill. Strain into 2 Martini glasses. Garnish with lime slices. Repeat 3 more times for 8 cocktails.