Slice the vanilla bean open lengthwise and scrape out the tiny seeds with the tip of a knife.
Combine the milk, sugar, and vanilla seeds (or vanilla extract) in a blender, and blend until the sugar dissolves completely. The mixture should be smooth and fragrant.
Add the ice cubes to the blender, filling it about halfway, and pulse a few times to start crushing the ice.
Blend on high until the mixture becomes thick, smooth, and slushy, with a frosty appearance and a gentle swirl pattern.
Taste the slushie and adjust sweetness or vanilla if needed, blending briefly to incorporate any additions.
Pour the chilled vanilla slushie into glasses, and serve immediately with a straw or a spoon for sipping.