Combine the cranberries and sugar in a medium saucepan over medium heat. Add the vanilla extract or split vanilla bean and cook, stirring, until the cranberries burst, 5 to 6 minutes.
Divide the cranberry mixture between 2 large mason jars. Divide the vodka between the jars. Cover and refrigerate for 1 week. Strain out the cranberries. Store the flavored vodka in a clean jar in the refrigerator.
For each drink, pour 2 ounces cranberry vodka over ice in a tall glass; top with tonic. Garnish with a slice of lime.
Photograph by James Wojcik
Recipe courtesy Michael Chiarello for Food Network Magazine