Place cranberries, sugar and vanilla in a medium saucepan. Place pan over medium heat and stir. Simmer cranberry mixture until the berries burst, about 5 to 6 minutes.
Divide mixture in half and pour into large, clean mason jars. Pour vodka into the jars to cover the berries. Set aside and allow to sit for 1 week. After 1 week, strain out the cranberries and store cranberry vodka in a clean jar in the refrigerator.
To serve: Pour 2 ounces of vodka mixture over ice in a tall glass and top with tonic. Garnish with a slice of lime.