Bring 12 cups water, the mint, tea, vanilla seeds and sugar to a boil in a large pot over high heat. Boil for about 10 minutes, then remove from the heat and let steep for another 10 minutes. Remove the tea bags and mint sprigs and strain into large pitcher. Let cool to room temperature, then refrigerate until ready to serve.
For each serving, fill a glass with ice, add 2 ounces of the bourbon and pour tea over. Garnish with a mint sprig mint and a lemon slice.