Mix the Ingredients: Pour the milk, Kahlua, vodka, and vanilla extract into a large bowl. Stir well until the mixture is smooth and evenly combined.
Set Up the Ninja SLUSHi: Place the clean machine on a level counter. Install the condensation catch, auger, vessel, bail handle, and drip tray according to the machine instructions. For the first use, let the machine sit upright for 2 hours so the refrigerant can settle.
Add the Mixture: Pour the White Russian mixture into the machine through the fill port. Close the cover and select the Spiked Slush setting.
Freeze the Slush: Let the Ninja SLUSHi run until the LEDs turn solid and the machine beeps. This usually takes about 15–60 minutes, depending on the starting temperature and desired texture.
Dispense and Serve: Place a glass under the dispensing handle and pull the handle forward. Release it to stop pouring. Serve the White Russian Slush right away for the thickest, smoothest texture.