In a large saucepan, combine the 4 cups cranberries, 4 cups water, ¾ cup sugar, and ¼ cup lemon juice. Bring to a boil over high heat, uncovered, stirring occasionally. Once the ingredients come to a boil, reduce the heat to low, cover the pan with a lid, and simmer for 20 minutes.
Let the pan sit uncovered for 5 minutes. Strain the cranberries through a fine-mesh strainer, pressing the cranberries into the strainer with a rubber spatula.
Allow the cranberry juice to cool for 10 to 15 minutes before stirring in the 1 ¾ cup vodka. Carefully pour the mixture into a plastic zip bag and place it in a flat bottom container in the freezer overnight or at least 8 hours.
Scoop into your favorite glassware and serve with the classic fresh lime garnish.
When straining the solids from the cranberry juice, press the cranberries into the mesh strainer and discard the solids as needed. Leftover cranberries skins make for a perfect simple jam and is great on toast!
You can store the slushie liquid in any container that works for you, the plastic storage bags make breaking apart the slush nice and easy when it first comes out of the freezer. It could also be scooped - like Italian ice - out of a container.