Add the vanilla ice cream, milk, vanilla extract, and sweetener (if using) into the blender.
Start blending on low speed to break up the ice cream, then increase to high for about 30 seconds until the mixture is smooth and creamy.
Pause the blender and check the texture; it should be thick and velvety, resembling a soft snow cone.
If the mixture is too thick, add a splash more milk and pulse briefly to incorporate.
Once blended to your desired consistency, turn off the blender and give the mixture a gentle stir with a spoon to ensure even flavor.
Pour the vanilla slushie into glasses, using a spoon or ladle, and serve immediately for a refreshing cold treat.